Instructions

For the cake

1. Preheat the oven to 180°C/356°F (without fan).
2. Line the bottom of a 23 cm/9 inch springform cake pan with baking paper. Do not grease the pan.
3. Melt the butter and chocolate together in a bain-marie (a heatproof bowl over a pan of simmering water) or in the microwave. Set it aside to cool slightly.
4. Separate the 4 eggs into two large mixing bowls.
5. Whisk the egg whites until they are thick and frothy.
6. Add the 75 g (1/3 cup) of sugar, one tablespoon at a time, and keep whisking until the egg whites become thick and glossy.
7. In the mixing bowl with the 4 egg yolks, whisk in the remaining 2 eggs, plus the remaining sugar and liqueur (if using). 
8. Whisk in the chocolate mixture.
9. Fold in the whisked egg whites, one-third at a time, taking care not to deflate the mixture.
10. Once the egg whites have been evenly incorporated into the batter, pour the batter into the cake pan.
11. Gently shake the pan to level out the batter.
12. Bake the cake for 30 to 40 minutes, or until the cake has risen and the centre does not wobble when you shake the pan gently. If you use a cake skewer, it will leave a visible hole in the centre of the cake, but this is ok if you plan to decorate the cake later.
13. Place the cake pan on a wire rack to completely cool and set. The cake will sink as it cools.
14. Use a small palette knife to carefully remove the cake from the sides of the pan.
15. Transfer to the cake to a plate for serving.

For the topping

1. Place the cream and liqueur (if using) into a large, clean bowl.
2. Whisk until the cream is thick and soft. Be careful not to over-whip the cream.
3. Dollop the cream onto the cake.
4. Dust with sifted cocoa powder or shaved/grated chocolate.