Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with parchment paper.

Sift together the all-purpose flour, cocoa powder, baking powder, and salt in a bowl.

Cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition, followed by the vanilla extract.

Gradually add the sifted dry ingredients to the wet mixture, mixing until just combined.

Pour the batter into the prepared cake pan and smooth the top. Bake for 20–25 minutes until a toothpick comes out clean. Let it cool in the pan for 10 minutes before transferring to a wire rack.

Melt the dark chocolate and butter together in a heatproof bowl over simmering water until smooth. Allow to cool slightly.

Whisk the egg yolks and granulated sugar in another bowl until pale and creamy. Stir in the melted chocolate mixture and vanilla extract.

Beat the egg whites in a clean bowl until stiff peaks form, then gently fold into the chocolate mixture.

Whip the heavy cream to soft peaks and fold into the chocolate mixture until combined.

Layer the cooled chocolate sponge cake on a plate and spread half of the mousse over it. Refrigerate for 30 minutes.

Top with the remaining mousse and smooth the surface, then refrigerate for at least 2 hours until set.

Serve chilled, slicing to reveal the layers of chocolate delight.