Swiss meringue buttercream

Swiss meringue buttercream is smooth, less sweet than icing-sugar frosting and very stable. It pipes well, it levels cleanly, and it doesn’t slump quickly at room temperature. People fear that it curdles, but almost always that is fixable: it’s a matter of temperature, not a ruined batch.

Ingredients and what each one does

• Egg whites make the foam. Whipped with sugar they become a strong meringue that carries the butter.
• Sugar dissolves as the whites are heated and stabilises the meringue. Fine sugar dissolves faster.
• Butter is flavour, richness and structure: it firms when chilled. It’s the largest ingredient, so the taste of the buttercream is the taste of the butter.

How to make it

1. Set the whites and sugar over a pan of barely simmering water and whisk until the sugar has dissolved completely. Test by rubbing a drop between your fingers (no grit) or by temperature, about 71 °C. This heat also makes the whites safer to eat.
2. Whip with a mixer to stiff peaks and keep going until the bowl has cooled to room temperature, 20–25 °C. Warm meringue melts the butter. It takes roughly 8–12 minutes, but the temperature of the bowl is the sign, not the clock.
3. The butter should be soft, around 18–22 °C. Add it in cubes, one at a time, on medium speed. At first the mix will collapse and look awful; then it comes together smooth.
4. Whip until glossy and smooth, then add vanilla or salt at the end.

What can go wrong

• Curdled, like cottage cheese. The butter was too cold and chilled the mix. This is the most common case. Keep whipping, or warm the side of the bowl with a hair dryer or torch for 5–10 seconds while the mixer runs.
• Soupy. The meringue was still warm, so the butter melted. Chill the bowl for 10–15 minutes and whip again.
• Gritty. The sugar never fully dissolved over the water bath.
• Greasy taste. The buttercream is too cold to serve; let it warm up a little.

Curdled and soupy buttercream is rarely ruined for good. Temperature fixes it, not extra ingredients.

Substitutions

Carton (pasteurised) whites work the same, though they may whip a little slower. You can cut the sugar a bit, but not by much, since it stabilises the meringue. Flavourings go in last and in small amounts: alcohol, syrups and acidic purées thin the buttercream and can split it.

Storage

Up to 5 days in the fridge, a month in the freezer. Bring it to room temperature and re-whip until smooth before use.