¼ cup milk
2 Tablespoons sweet butter, cut into small pieces
¾ cup sifted cake flour
1 teaspoon baking powder
¾ cup sugar
3 large eggs
3 egg yolks
Preheat oven to 400F. Line the bottom of a sheet pan with parchment of wax paper so that the paper overhands the pan at opposite ends.
Heat milk with butter in a small pan until butter melts. Keep hot but do not simmer.
Sift flour with baking powder and set aside.
In the bowl of an electric mixer whisk sugar, whole eggs and egg yolks. Set bowl over or in a pan of barely simmering water. Heat mixture, whisking occasionally until lukewarm to touch. Beat in mixer until tripled in volume and has the consistency of thick whipped cream. Fold in flour mixture 1/3 at a time.
Pour in the hot milk and butter into batter and fold well. Pour into prepared pan. Bake about 10 minutes.