Creaming butter and sugar

Why

When soft butter is beaten with granulated sugar, the sharp edges of the crystals cut through the fat and leave tiny air bubbles behind, which the firm but pliable butter holds; in the oven, steam and carbon dioxide from the baking powder move into them and the batter rises, so a well-creamed mixture gives a fine, fluffy crumb. The temperature of the butter decides it: according to America's Test Kitchen it should be about 18–19 °C, when it bends without cracking and gives slightly under a finger. Cold butter takes in air only slowly, and butter that is too soft or melting cannot hold it, so the bubbles collapse and the cake is dense and greasy. Eggs form an emulsion with the butter; cold eggs firm the butter into tiny lumps, the mixture curdles and some of the air escapes, which is why eggs go in at room temperature and one at a time. Icing sugar is so fine that it usually works less air into the butter, which does not matter in crisp biscuits but tends to give a denser crumb in a leavened creamed cake.

How to do it

• Take the butter out of the fridge about 30 minutes ahead, earlier in a cold kitchen; softened butter bends and gives under a finger but is not greasy or shiny.
• Beat the butter with granulated sugar at medium speed for about 3–5 minutes, until the mixture turns pale, fluffy and grows in volume.
• Use eggs at room temperature; cold ones warm up in about 10 minutes in their shells in a bowl of warm water.
• Add the eggs one at a time and beat each one in until smooth before adding the next.
• Fold in the flour last and only briefly, until no dry patches remain; long mixing drives out air and strengthens gluten (see the Kneading and gluten card).
• If the butter turns shiny and starts to run while beating, put the bowl in the fridge for 10 minutes and then carry on.

If it goes wrong

A mixture curdled by cold eggs can be saved: set the bowl over warm water for a moment and beat until it comes together again, or beat in 1–2 tablespoons of the recipe's flour; if it curdled because the butter was too soft, chill it for 10 minutes instead. A slightly curdled mixture usually comes together once the flour goes in and the cake still bakes, just a little denser. Let butter that has melted firm up in the fridge to a soft consistency and cream it with the sugar again; melted butter will never give a fluffy mixture.