Preheat the oven to 180C/350F/Gas 4. Grease three 20cm/8in round cake tins with butter and line with baking paper (or use non-stick tins).
For the sponge, melt the chocolate in a heatproof bowl set over a saucepan of simmering water, stirring frequently. When melted, remove from the heat and set aside for the chocolate to cool a little.
In a mixing bowl, beat the butter with the sugar for 10 minutes until soft. Mix in the beaten eggs bit by bit, then gently fold in the flour, baking powder and cocoa. If the mixture feels stiff, add some milk to slacken it. Stir in the cooled runny chocolate, then tip the mixture into the prepared tins. Bake for 25–30 minutes, or until the cakes are risen and springy. Leave to cool a little in the tins, then transfer to a wire rack to cool fully.
Meanwhile, make the peanut brittle. Line a baking tray with greaseproof paper. Melt the sugar in a frying pan over a high heat for a few minutes, keeping a close eye on it, until it becomes a golden caramel. Remove from the heat and stir in the peanuts, then pour immediately onto the lined baking tray. Allow to cool completely and then break into chunks.
For the frosting, beat the peanut butter with the icing sugar and butter until smooth. Beat in the soured cream, taste, then adjust the sweetness if necessary with a little more icing sugar.
When the cakes have cooled, place one on a plate or cake stand and spread with some of the frosting. Top with another cake layer, more frosting, than the final layer of cake. Frost the top of the cake, using a piping bag if you wish. Finally, decorate with shards of the peanut brittle.