Hot Water Sponge Cake

Ingredients

• 2 egg yolks
• 1 cup sugar
• 3/8 cup hot water or milk
• 1/4 teaspoon lemon extract
• 2 egg whites
• 1 cup flour
• 1.5 teaspoons baking powder
• 1/4 teaspoon salt

Method

Beat the egg yolks at high speed until they triple in volume and pale to a cream colour — this takes four to five minutes with an electric mixer, longer by hand. The yolks are being whipping|whipped into a stable foam that will give the cake its characteristic open crumb. Whisk in half the caster sugar gradually, continuing to beat. Once incorporated, pour in the hot water or milk in a thin stream whilst beating, then add the remaining sugar and lemon extract. The hot liquid does not collapse the foam as you'd expect — instead, it loosens the batter whilst the continued beating maintains the emulsification of yolk fat into the water-sugar mixture. This paradox is the crux of the sponge-cake; thermal agitation keeps the emulsion stable.

Whip the egg whites to stiff peaks in a separate, scrupulously clean bowl. Any trace of yolk fat will prevent them from rising, so either separate them carefully or use fresh whites. Fold the whites into the yolk mixture using a spatula and a cutting motion, rotating the bowl as you work. Fold only until no white streaks remain — overworking knocks air out of both foams.

Sift the flour, baking powder and salt together, then fold this dry mix into the batter in two additions, using the same gentle folding technique. Work quickly once the flour hits the wet batter; the leavening agents begin reacting with the moisture immediately.

Pour into a buttered and floured shallow tin — a 23-centimetre square or similar. The batter should be thin and pourable. Bake at 190°C for twenty-five to thirty minutes. The cake is done when a fine skewer inserted at the centre withdraws clean and the top springs back when pressed lightly near the edge. The surface should be pale golden, not deep brown; this cake depends on the baking of the foam itself, not on crust development.

Cool briefly in the tin, then turn out onto a wire rack. Serve at room temperature, optionally dusted with caster sugar or layered with jam and whipped cream.