Preheat the oven to 170-180°C (340-355°F).

Grease a 24 cm (9-inch) springform pan with butter for easy removal.

Melt the chocolate in a double boiler, then stir in the butter until smooth and glossy.

Whisk sugar and egg yolks together in a large bowl until pale and doubled in volume.

Mix in the melted chocolate-butter mixture until fully combined.

Whip the egg whites with a pinch of salt until stiff peaks form.

Stir in a spoonful of egg whites into the chocolate mixture to loosen the batter.

Gently fold in the remaining egg whites with a spatula to keep the mixture light and airy.

Pour half the batter into the pan and smooth the top.

Bake for 45 minutes, checking for doneness (a knife should come out clean).

Let the cake cool completely, then spread the remaining uncooked batter over the baked base.

Chill for at least 12 hours in the fridge to allow the mousse to set.

Remove the ring from the springform pan gently.

Dust with unsweetened cocoa powder and decorate the sides with chocolate-coated rolled wafer cookies for a festive finish.