A proper Irish coffee is hot black coffee, a teaspoon of brown sugar, 40ml of Irish whiskey and a layer of lightly whipped cream floated on top — never stirred in. Dissolve the sugar in the hot coffee first, add the whiskey, then pour the cream slowly over the back of a warm spoon so it settles on the surface. You drink the hot, sweet, whiskey-laced coffee straight through the cool cream, and that contrast is the entire point of the drink.

It is one of the very few cocktails Ireland can fairly claim to have invented on home soil, and one of the easiest in the world to get wrong. Four ingredients, five minutes, and about three places where people come unstuck. Here is how to get it right every time.

What goes into a proper Irish coffee?

• 150ml hot black coffee — freshly brewed and genuinely hot. Filter coffee or a strong cafetière is ideal. A double espresso lengthened with hot water works too.

• 1 teaspoon brown sugar — demerara or soft brown. The sugar is not only for sweetness; it is a structural ingredient, and we will come back to why.

• 40ml Irish whiskey — one measure. A smooth, characterful blend that can stand up to coffee without vanishing into it.

• A small layer of cream — fresh double or whipping cream, whipped just until it thickens and barely holds a ribbon. Not stiff. It has to pour.

• A warmed, stemmed glass — a classic Irish coffee glass, or any small heatproof tumbler.

What is the correct Irish coffee ratio?

The working ratio is roughly four parts coffee to one part whiskey, with a teaspoon of sugar and a finger of cream on top. In practice that is 150ml of coffee to 40ml of whiskey, which fills a standard six-ounce Irish coffee glass with just enough headroom left for the cream.

If your glass is smaller, scale the whole thing down rather than squeezing the coffee: 120ml coffee to 30ml whiskey keeps the balance intact. What you should not do is stretch the whiskey. Two measures makes a hot toddy wearing a cream hat, not an Irish coffee. The coffee is meant to lead.

How do you make an Irish coffee?

1. Warm the glass. Fill it with boiling water, leave it while you brew, then tip it out and dry it. A cold glass drops the temperature of the drink instantly and it never recovers.

2. Sugar first, then coffee. Add the brown sugar to the warm glass and pour in the hot coffee. Stir until every last grain has dissolved. Undissolved sugar is the single most common reason the cream sinks.

3. Add the whiskey. Pour in your measure and give it one gentle stir. Leave a couple of centimetres of headroom.

4. Float the cream. Whip the cream lightly, until it just thickens and starts to move slowly in the bowl. Hold a warm spoon upside down just above the surface of the coffee and pour the cream over the back of it so it spreads out and settles.

5. Do not stir it. Serve it as it is. No spoon, no straw — you drink the hot coffee through the cool cream.

Why won't the cream float on my Irish coffee?

Almost always one of three things.

The sugar has not fully dissolved. Sweetened coffee is denser than unsweetened coffee, and that density difference is what holds the cream up. Skip the sugar, or leave grains sitting in the bottom, and the cream has nothing to rest on.

The cream is wrong. Too runny and it cuts straight through; whipped stiff and it sits in a lump rather than a layer. You want it thickened just to the point where it slows down on the spoon and still pours freely.

You poured too fast. The back of a warm spoon is not decoration — it breaks the fall of the cream so it lands flat instead of punching a hole through the surface. Pour slowly, close to the surface, and give it a second to spread.

The Buena Vista in San Francisco, which has been making these by the thousand since 1952, famously solved the problem by ageing its cream for a couple of days before whipping, which thickens it naturally. You do not need to go that far at home. Sweet coffee, lightly whipped cream, slow pour.

What coffee should you use?

A medium-roast filter coffee is the safest choice: enough body to carry the whiskey, not so much bitterness that it starts fighting the cream. Dark, oily espresso roasts can turn harsh once sugar and cream arrive, and very light roasts tend to taste thin underneath everything else.

Brew it a little stronger than you would drink it black. The whiskey, sugar and cream all soften the coffee, so a cup that tastes slightly assertive on its own ends up balanced in the glass. And brew it fresh — coffee that has been sitting on a hot plate brings a stewed note that no amount of cream will hide.

What whiskey should you use in an Irish coffee?

Use an Irish whiskey smooth enough that you would happily sip it neat, but with enough character to come through coffee and cream. Triple-distilled blends are the natural fit: they are rounded and approachable, and they sit alongside the roast instead of arguing with it.

Single malts can work, but a lot of what makes a good malt interesting gets buried under coffee. If you have a bottle you love drinking on its own, save it for the glass on its own.

The one genuine mistake is reaching for something harsh on the assumption that the coffee will hide it. It will not. Whatever edge is in the bottle turns up in the cup, warmed and amplified.

At Flying Tumbler, our pick for an Irish coffee is The Roller — a smooth, easy-going blend that carries warmth and a little sweetness right through the cream without being shy about it. It is the bottle we reach for when the evenings start drawing in.

If you want to take it somewhere of its own, make a Solstice Coffee: our signature winter twist on the standard build, with a strip of orange peel and a small pinch of cinnamon added at the sugar step, made with The Roller. It is the version we make when there is frost on the window and someone has just come in from the cold.

Who invented Irish coffee?

Joe Sheridan, a chef at the flying-boat terminal at Foynes in County Limerick, in the winter of 1943. A transatlantic flight turned back in bad weather, the passengers came ashore cold and miserable, and Sheridan put whiskey in their coffee to warm them up, sweetened it and floated cream on top. When one of them asked whether it was Brazilian coffee, he told them it was Irish coffee. The name stuck.

It reached America in 1952, when the travel writer Stanton Delaplane and the owner of the Buena Vista café in San Francisco set out to reproduce it — and spent a long time failing at the cream before they got it right. The Buena Vista has been pouring them ever since.

So it is a genuinely Irish invention, and, fittingly, one that was invented for people arriving.

What's the difference between Irish coffee, Gaelic coffee and Baileys coffee?

Irish coffee is coffee, sugar, Irish whiskey and floated cream. Gaelic coffee is, in most places, the same drink under a different name — though some bars use the term for a version made with Scotch, which makes it a Scottish coffee by any sensible reading.

Baileys coffee is a different animal. That is coffee with Irish cream liqueur stirred through it, so the cream and the sweetness are already in the bottle and there is no float at all. It is pleasant enough, but it is a creamy coffee rather than a layered cocktail — you lose the hot-through-cold contrast that makes an Irish coffee what it is.

Can you make Irish coffee for a crowd?

You can, and it is one of the better things to hand people at a winter gathering — but do it in a way that does not chain you to the kettle all night.

Brew a full pot of coffee and keep it hot. Warm all the glasses together in the sink with boiling water. Whip a bowl of cream to the right consistency in advance and keep it cold; it will hold for an hour or so. Then it is sugar, coffee, whiskey, cream, four glasses at a time.

The one step you cannot batch is the cream float. Pour each one individually. It takes ten seconds and it is the whole show.

Variations worth trying

• Iced Irish coffee — sweeten and cool the coffee, pour it over ice with the whiskey, and float the cream on top. A summer answer to a winter drink.

• Solstice Coffee — orange peel and a pinch of cinnamon at the sugar step, as above.

• Maple instead of sugar — a teaspoon of maple syrup gives a rounder, deeper sweetness and dissolves instantly.

• Decaf, same ritual — a proper Irish coffee works perfectly well on good decaf if it is a late one.

A drink with a homecoming in it

Irish coffee was built for arrivals: for people who had come a long way in bad weather and needed something warm at the end of it. That is a story we rather like at Flying Tumbler, given that our whole world turns on the journey out and the welcome home.

Make one for someone who has just walked through the door. It is a small ritual, and it lands every time.

Flying Tumbler is a family-owned Irish whiskey brand from County Carlow. Please enjoy responsibly. 25+.