Cocktails with Double cream/heavy cream
Double cream (UK) or heavy cream (US) is the richest liquid dairy cream, at least 36% milkfat in the US or 48% in the UK.
How it's used
Cream gives a drink body and turns it opaque; lightly whipped, it floats as a cap. Fat does the work, and UK double cream has about a third more of it than US heavy cream. The same measure therefore gives a noticeably richer drink. Keep the shake short: a long, hard shake churns cream toward butter.
Substitutes for Double cream/heavy cream
The closest swap, in the same amount. A shaken cocktail turns out slightly thinner, and it still whips for a float. UHT cartons whip less readily.
Single or pouring cream
Fine for shaking, with a thinner result. It will not work for a float because it cannot whip enough.
Storage
Keep it sealed in the fridge, away from strong smells, and use it by the date. Don't leave it at the back of a warm station between drinks.
