Preheat the oven to 180°C/fan 160°C/ gas 4. Take two 23cm (9in) diameter tins with sides at least 6cm (2½in) high. Butter the sides of each cake tin and dust with flour. Line the base of each tin with a disc of baking parchment. Melt the butter and set aside to cool. Whisk the eggs and sugar in a bowl until thick. Sift in the flour, cocoa powder, baking powder and ¼tsp of salt, then fold in the vanilla extract and the melted butter.
Quickly divide the batter between the tins and bake for 30 minutes. Remove from the oven, cool, then remove and transfer to a wire rack. Melt the chocolate in a heatproof bowl set over a pan of simmering water.
Remove from the heat. Beat in the egg yolks one at a time, then beat in the butter, spirits or vanilla extract until smooth. Cool. In a clean bowl whisk the egg whites until they form stiff peaks. Fold in a quarter of the chocolate mixture, then carefully fold in the rest. Cool at room temperature.
Use a bread knife to carefully slice each cake in half horizontally. Place the bottom half of one of the cakes, cut side up, on a plate. Spread over some of the mousse, then sandwich with the other half of the cake, placing it cut side down.
Spread over more of the mousse, then sandwich with the bottom half of the second cake, cut side up. Repeat for the next layer until you have four layers of cake sandwiched together with three layers of mousse. Cover the top and sides of the cake with the remaining mousse, smoothing the whole over with a palette knife. Top with chocolate curls, or dust with cocoa powder.