Preheat the oven to 170C/Gas 3/fan oven 150C. Grease a 23cm/9in springform cake tin and line the base with greaseproof paper or baking parchment. Dust the sides with a little flour. Put the chocolate and butter into a heatproof bowl set over a pan of gently simmering water. Heat until melted, then remove the bowl from the pan and stir until smooth. Leave for about 5 minutes to cool slightly.
Stir in the egg yolks, ground almonds, and the liqueur, if using. Put the egg whites into a large bowl, add a pinch of salt and whisk until soft peaks form. Continue whisking, sprinkling in the sugar a little at a time, until stiff peaks form. Stir 2 tablespoons of the whites into the chocolate mixture, then carefully fold in the remainder until no
traces of white are left.
Spoon the mixture into the prepared tin and bake for 30-35 minutes until well risen and just firm to the touch. Cool in the tin (don’t worry if the cake sinks and cracks slightly – it will still be fine). You can make the cake up to this stage up to four days ahead and keep it in an airtight container in a cool place. It also freezes well for up to one month.
To serve, remove the cake from the tin and peel away the lining paper. Sift cocoa powder liberally over the top and cut into slices. Serve with crème fraîche.