Sour cream frosting is the simplest cake filling: sour cream and sugar. Nearly every failure is the same, it turns out runny and slides off the layers. The cause is not “bad sour cream” but water: sour cream contains a lot, and you must either remove it or bind it.
Ingredients and what each one does
- Sour cream at 25% fat or more. Fat gives body when chilled, water takes it away. Low-fat sour cream (10 to 15%) usually stays runny.
- Sugar dissolves into the water of the sour cream and makes it thinner at first. The cream thickens later with whipping and chilling. Powdered sugar dissolves faster and leaves no grit.
How to make it
- If the sour cream is watery, drain it: put it in a sieve lined with cheesecloth in the fridge for 3 to 12 hours. The whey that drips out is the surplus water.
- Keep the sour cream cold and use fine or powdered sugar. Whip on medium speed until the cream thickens and holds a trail from the whisk. Sign: a ribbon falls from the spatula and does not spread at once.
- Do not overwhip. The fat can start to separate and the cream turns grainy, with tiny lumps and whey.
- Chill 30 to 60 minutes before assembling. It firms up further in that time.
What can go wrong
- Runny. The sour cream was too lean or undrained. Fix it by draining, not by adding flour or starch, which only taste raw.
- Grainy with whey. Overwhipped. It cannot be undone, so use it in a layer and eat it sooner.
- Too tangy. Add more sugar, or fold in cream whipped separately.
- Slides off the cake. The sponge is wet or the cream is warm. Spread it cold.
Substitutions
Cold whipped cream (33 to 35% fat) folded into sour cream gives a lighter, more stable frosting. Strained Greek yoghurt can replace sour cream but is tangier. Condensed milk instead of sugar gives a caramel note and a looser cream.
Storage
In the fridge for 2 to 3 days. A cake with sour cream frosting improves after 6 to 12 hours: the sponge takes up moisture from the cream, which thickens further.
