1. Make the sponge cake: Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. In a small bowl, whisk together flour, baking powder, and salt. In a large bowl, beat eggs and sugar until thick and pale. Heat milk and butter until just warm; stir in vanilla. Gently fold dry ingredients into egg mixture, then fold in warm milk mixture. Divide batter between pans and bake 20–25 minutes, until golden and springy. Cool completely.

2. Make the pastry cream: In a saucepan, heat milk until steaming. In a bowl, whisk sugar, egg yolks, and cornstarch until smooth. Slowly whisk hot milk into yolk mixture, then return all to saucepan. Cook over medium heat, whisking constantly, until thickened. Remove from heat, stir in butter and vanilla, and let cool with plastic wrap pressed directly on the surface. Chill until firm.

3. Make the glaze: Heat cream until just simmering. Pour over chopped chocolate and butter. Let sit 2 minutes, then stir until smooth and glossy.

4. Assemble: Place one cake layer on a platter. Spread pastry cream evenly on top. Add second cake layer. Pour warm chocolate glaze over the top, letting it drip slightly down the sides. Chill briefly to set before serving