This chocolate cake is extremely fudgy! Feel free to swap the ground almonds for ground hazelnuts.
Preheat the oven to 350°F and coat a 20cm/8″ diameter spring-form tin with butter and cacao powder.
Melt the chocolate and butter in a large heatproof bowl sitting over a saucepan of barely simmering water. Remove the bowl from the heat and mix in the sugar, vanilla extract and ground almonds with a whisk. Stir through the egg yolks, one at a time, mixing after each addition, until you have a thick batter.
Whisk the egg whites until stiff peaks form. Add the egg whites to the chocolate batter and fold through gently until just combined.
Pour the chocolate batter into the prepared cake tin and place in the oven to bake for about 35 minutes until it is firm but with a slight wobble. Remove the cake from the oven and allow it to sit on a wire rack to cool in its tin, before removing the tin to cool completely.