Set the oven to 175ºC. Grease and crumb a large, round cake tin.
Melt the fat in a saucepan. Add the crumbled biscuits and leave to cool.
Melt 125 g dark chocolate together with 2 tbsp water in a saucepan whilst stirring.
Chop the remaining 125 g dark chocolate and the walnuts.
Mix the cane sugar, bicarbonate of soda, flour, sour milk, eggs and the melted chocolate in a bowl, and whisk to give a smooth batter.
Mix the chopped walnuts and chocolate into the biscuit crumbs.
Pour the cake batter into the tin, and then pour over the biscuit crumb mixture which will sink into the batter.
Bake the chocolate cake in the middle of the oven at for approx. 50 minutes. Dust with icing sugar to serve.
The recipe was made by