Boston Cream Pie

For the sponge, beat 3 eggs with sugar for at least 5 minutes, until the mixture is pale, thick, and falls from the whisk in ribbons. Sift in the cake flour and fold from the bottom with a light hand so the foam retains its volume. Bake the batter in an 18cm round pan at 175°C for 25 to 30 minutes, checking that the center springs back and a skewer comes out clean. The cake must cool completely before it is split horizontally. Make the milk custard by whisking milk, sugar, and cornstarch until smooth before the pan goes over the heat. Stir constantly over medium heat until the cream becomes thick enough for the whisk to leave clear tracks across the bottom. Transfer it to a bowl, press plastic wrap directly against the surface, and chill it until cold and firm. Spread an even, generous layer over the lower sponge, then align the upper layer over the filling. Melt the dark chocolate and unsalted butter together over a double boiler. Let the glaze cool until slightly warm, pour it over the top, and refrigerate the assembled cake for 30 minutes. Once the coating has set, a clean slice reveals the separate sponge, custard, and chocolate layers without the filling running out at the sides.