Lemon Sponge Cake
Cake flour is milled from soft wheat and, as a result, contains less gluten and more starch than all-purpose flour. Because cake flour is milled finer than other flours, its particles are small, allowing it to blend into a batter easily. Cake flour is also bleached, which enables it to tolerate the high proportion of sugar and fat in cake batters. All these characteristics produce a tender cake with a fine crumb, making cake flour appropriate for this lemon sponge and other delicate cakes.
For the cake:
- 5 eggs, at room temperature
- 3/4 cup sugar
- 3/4 tsp. lemon extract
- 1 cup cake flour, sifted
- 4 Tbs. (1/2 stick) unsalted butter, melted and cooled to room temperature
For the sugar syrup:
- 1/4 cup sugar
- 1/4 cup water
For the lemon buttercream:
- 24 Tbs. (3 sticks) unsalted butter, at room temperature
- 1/2 cup milk
- 3/4 cup sugar
- 5 egg yolks
- 1 Tbs. lemon extract
- 1/2 cup heavy cream
- 1 cup Lemon Curd
Preheat an oven to 375°F. Line the bottom of a 9-by-3-inch round cake pan with parchment paper.
To make the cake, in the bowl of an electric mixer fitted with the whisk attachment, beat the eggs and sugar on high speed until tripled in volume, about 5 minutes. Beat in the lemon extract. Remove the bowl from the mixer. Sift the flour over the egg mixture in 2 additions and carefully fold in with a large rubber spatula. Fold a large dollop into the melted butter, then fold back into the egg mixture. Pour into the prepared pan and smooth the top. Bake until the cake is puffed, 20 to 25 minutes. Transfer the pan to a wire rack and let cool completely.
To make the sugar syrup, in a small saucepan over medium heat, combine the sugar and water and bring to a boil, stirring occasionally until the sugar dissolves. Remove from the heat and let the syrup cool to room temperature.
To make the lemon buttercream, in the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until it is the consistency of mayonnaise; it should not be melted. Transfer the butter to another bowl. Thoroughly wash and dry the mixer bowl. In a saucepan over medium heat, combine the milk and 1/4 cup of the sugar and heat, stirring occasionally, until small bubbles appear along the edge of the pan.
Meanwhile, in the bowl of the electric mixer fitted with the whisk attachment, beat the egg yolks and the remaining 1/2 cup sugar on medium-high speed until the mixture is pale and thick, about 3 minutes. Reduce the speed to low and pour in the hot milk mixture in a thin stream. Return the mixture to the saucepan. Thoroughly wash and dry the mixer bowl and whisk.
Cook the mixture over medium heat, whisking constantly, until it registers 170°F on an instant-read thermometer, 5 to 7 minutes. Pour the mixture back into the mixer bowl and beat with the whisk on medium speed until cool, 5 to 10 minutes. Beat in the lemon extract. Add the butter in 4 additions, incorporating each addition before adding another. Use the buttercream immediately, or refrigerate until needed, up to 3 days. If the buttercream has been refrigerated, whisk it by hand over simmering water until it reaches spreading consistency.
Whip the cream to soft peaks. Put the lemon curd in a bowl and carefully fold in the whipped cream in 2 additions.
Run a table knife around the edge of the cake pan and unmold the cake onto a work surface. Turn the cake right side up, leaving the parchment paper in place. Cut the cake into 2 equal layers. Put the top layer, cut side up, on a serving plate. Brush with some of the sugar syrup. Reserve one-third of the buttercream for decorating the frosted cake. Fit a pastry bag with a 1/2-inch plain tip, fill the bag with about 3/4 cup of the remaining buttercream and pipe a ring around the outside edge of the cake. Evenly spread the whipped cream mixture inside the buttercream ring. Position the second layer, cut side down, on the cream and peel off the paper. Brush with the remaining syrup. Refrigerate for 30 minutes; keep the remaining buttercream at room temperature.
Spread buttercream on the top and sides of the cake. Fit the pastry bag with a 1/2-inch star tip, fill the bag with the reserved buttercream and pipe shells around the top edge. Refrigerate the cake until 30 minutes before serving. Serves 10 to 12.