Preheat oven to 350°F. Grease a 9-inch springform pan and line the bottom with parchment. Whisk flour, cocoa powder, baking powder, and salt in a bowl. In a separate bowl, beat melted butter and sugar until combined, then beat in egg, milk, and vanilla. Fold in dry ingredients until just smooth. Spread batter evenly in pan and bake 15-18 minutes until the top springs back when lightly pressed. Cool completely in pan on a wire rack. Do not remove from pan.

Combine chocolate and butter in a heatproof bowl set over simmering water. Stir occasionally until completely smooth, about 5 minutes. Remove from heat and let cool slightly, 5 minutes. Whisk in egg yolks one at a time until glossy.

Beat egg whites with 2 tablespoons sugar on medium-high speed until soft peaks form, about 3 minutes. The peaks should curl over when you lift the beaters. Fold one-third of whites into chocolate to lighten, then gently fold in remaining whites until no streaks remain.

In a clean bowl, beat heavy cream with remaining 2 tablespoons sugar and vanilla on medium-high until medium peaks form, about 4 minutes. Fold cream into chocolate mixture in two additions, cutting through the center and turning the bowl until fully incorporated and mousse is uniform in color.

Pour mousse over cooled cake in the springform pan, smoothing the top with an offset spatula. Tap pan gently on counter to release air bubbles. Refrigerate uncovered until set, at least 4 hours or overnight. The mousse should feel firm to the touch.

Run a thin knife around the edge of the pan, then release the springform sides. Transfer to a serving plate. Dust with cocoa powder just before serving and top with raspberries if desired. Slice with a warm, dry knife for clean cuts.