Hot Milk Sponge Sheet
Preheat oven to 400 degrees. Line bottom of pan with waxed or parchment paper so that the paper overhangs the pan at two opposite ends. Heat milk with butter in small saucepan over medium heat until butter melts. Reduce heat to low and keep hot but do not simmer. Sift flour with baking powder twice. Return to sifter and set aside. In a large heatproof bowl, use a hand-held whisk to combine sugar, eggs and yolks. Set bowl over or in a pan of barely simmering water. Warm the mixture, whisking occasionally, until it is lukewarm to the touch. (Alternatively, omit the water bath and hold the bowl directly over the stove burner, whisking vigorously for about a minute until the eggs are warm. This is the quickest, easiest method but you must whisk the eggs continuously to keep them from starting to scramble). Transfer bowl to electric mixer; beat at high speed until mixture has cooled, tripled in volume and has the consistency of thick whipped cream. Sift one-third of flour mixture over batter and fold it gently, by hand, using the largest rubber spatula you have. Fold in half the remaining flour; then fold in the remaining flour. Pour hot milk and butter into batter and fold well, scraping the bottom each time and bringing the batter up the sides of the bowl until you can no longer see traces of liquid. Transfer batter into prepared pan and spread evenly. Bake until cake is browned, starts to shrink from the sides of pan, and springs back when lightly pressed: about 10 minutes. Cool cake in pan, on a rack. When cake is cool, run a small knife or spatula around the edges to release before unmolding. Cool completely before filling.