Preheat the oven to 350°F/ 4 and line an 8" diameter spring-form tin with baking parchment.
For the mud pie, 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 spatula. Stir through the egg yolks, one at a time, mixing after each addition, until you have a thick batter.
Put the egg whites in a standing food mixer (or use an electric hand-held mixer in a bowl) and 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.
Melt the chocolate and butter in a bowl sat over a pan of just simmering water. When melted, remove from the heat, whisk in the icing sugar and the cream. Allow to cool until the mixture becomes thick enough to leave a figure of eight on the surface. Pour over the chocolate mud pie and then sprinkle with hazelnuts. Leave to set before slicing and serving.
20cm/8″ diameter spring-form tin
Large heatproof bowl
Standing food mixer (or use an electric hand-held mixer in a bowl)