01 - Preheat oven to 350°F. Grease and line the base of a 9-inch springform pan with parchment paper.
02 - Sift together flour, cocoa powder, baking powder, and salt in a bowl.
03 - Beat eggs and sugar with an electric mixer until pale and tripled in volume, about 5 minutes.
04 - Gently fold in the dry ingredients. Drizzle in milk, oil, and vanilla extract; mix gently until just combined.
05 - Pour batter into the pan and bake for 18–20 minutes until a toothpick inserted in the center comes out clean. Cool completely in the pan.
06 - Melt chocolate in a heatproof bowl over simmering water, stirring until smooth. Cool slightly.
07 - Combine sugar and water in a small saucepan. Heat until sugar dissolves and mixture comes to a simmer.
08 - Beat egg yolks in a separate bowl. Gradually pour the hot syrup into the yolks while whisking constantly until pale and thick.
09 - Stir the melted chocolate and vanilla extract into the egg yolk mixture. Add a pinch of salt.
10 - Whip cold cream in a clean bowl to soft peaks.
11 - Fold one-third of the whipped cream into the chocolate mixture to lighten, then gently fold in the remainder until smooth.
12 - Remove cooled sponge from pan and place on a serving platter. Clean and reassemble the pan ring around the sponge.
13 - Pour mousse over the sponge, smoothing the top. Refrigerate for at least 4 hours until set.
14 - Heat cream until just simmering, then pour over chopped chocolate and butter. Let sit 1 minute, then stir until glossy and smooth. Cool for 10 minutes.
15 - Remove cake from pan. Pour glaze over the mousse layer, allowing it to drip down the sides. Smooth with a spatula. Chill for 30 minutes to set the glaze before serving.