Preheat the oven to 180°C, fan 160°C, gas 4. Grease a 25cm round springform cake tin.
Melt the chocolate and butter together in the microwave on High for 1-3 minutes, depending on your microwave, or melt over a pan of simmering water on the hob, taking care not to let any water come into contact with the chocolate. Stir in the vanilla extract.
Beat the egg yolks with 50g caster sugar until pale in colour, then pour in the chocolate mixture. Fold in the almonds and cocoa powder.
Whisk the egg whites until stiff, then gradually add the remaining 100g caster sugar until combined. Fold one-third of the egg white into the chocolate mixture, then gently fold in the rest until just blended.
Spoon into the tin, spreading evenly, and bake for 30-35 minutes. Let the cake stand for 10 minutes before dusting with icing sugar.
Cook's tip: turn this into a decadent dessert by serving with whipped mascarpone cheese sweetened with icing sugar.