Cream secrets: why frosting splits or runs, and how to make a stable cake cream

Almost every cream failure is a temperature failure. A cream is an emulsion: fat and water held together as one mass. If one component is cold and another warm, the fat either sets into little grains or melts, and the emulsion breaks. That's how you get a curdled, cottage-cheesy cream — or one that slides off the cake.

So before you start, always ask: what temperature should each ingredient be? The answer differs for each cream, and we'll go through them one by one.

Cream whips thanks to its fat: the fat droplets line up around air bubbles and hold them. With too little fat there's nothing to hold them — 10–20% cream won't whip however hard you try.

The second condition is cold. Warm fat is soft and can't support the foam. Chill the cream, the bowl and the whisk.

Plain whipped cream sinks and weeps after a few hours. For a cake that will sit in the fridge, it needs stabilising.

The easiest way is mascarpone or cream cheese. It adds richness and body, and the cream holds its shape for more than a day. The second way is gelatin: reliable, but it needs care or you'll get strings in the cream.

The most common complaint is runny cream-cheese frosting. The cause is almost always the same: the cheese was beaten too long. Cream cheese loses its structure with long beating and turns liquid, and you can't bring it back.

The right technique: first beat soft butter with icing sugar until fluffy, then add cold cream cheese straight from the fridge and beat for just 30–60 seconds — only until smooth.

The other popular version is made with cream instead of butter: cold cheese, cold 33–35% cream and icing sugar whipped together until thick. It's lighter and less sweet.

Pastry cream is thickened with starch and yolks. There's a non-obvious catch that makes it go runny a couple of hours later in the fridge. Egg yolks contain an enzyme that breaks down starch. If you take the cream off the heat the moment it thickens, the enzyme survives and 'eats' the thickening overnight.

So once the cream has thickened, boil it for 1 more minute, whisking constantly. Don't worry about the yolks curdling: the starch protects them, so the cream can take the boil.

Buttercream (butter with condensed milk, syrup or pastry cream) splits when the components are at different temperatures. The butter should be soft, around 18–20 °C (65–68 °F): a finger leaves an easy dent, but the butter isn't shiny or melting. Add the condensed milk or pastry cream at the same room temperature, a spoonful at a time.

If it splits anyway, look at it. Little grains with liquid at the bottom — it's too cold: warm the sides of the bowl with a hair dryer for 10–15 seconds, or dip the bowl in hot water for a few seconds, and keep beating. Runny and glossy — it's too warm: chill it for 10–15 minutes and beat again.

Sour-cream frosting runs if the sour cream isn't rich enough or has been overbeaten. Use sour cream with at least 20–25% fat and don't beat it for more than a couple of minutes.

The main secret is draining the excess whey. Tip the sour cream into a sieve lined with 2–3 layers of cheesecloth and leave it in the fridge overnight. Up to a third of the liquid drains off, and the sour cream becomes thick, almost like cream cheese. Frosting made from it holds its shape.

What ruins cream most often: