Preheat the oven to 180°C.
Grease and line three 20cm round cake tins.
Melt the dark chocolate in a heatproof bowl over simmering water and allow it to cool slightly.
Beat 350g butter with 350g sugar for 10 minutes until soft.
Mix in the beaten eggs gradually, then fold in the flour, baking powder, and cocoa powder.
Stir in the melted chocolate and divide the batter between the prepared tins.
Bake for 25–30 minutes until risen and springy.
Cool the cakes in the tins briefly before transferring to a wire rack.
Melt 150g sugar in a pan until it becomes a golden caramel, then stir in the peanuts.
Pour the mixture onto a lined tray, cool completely, and break into brittle shards.
Beat the peanut butter, icing sugar, and 70g butter until smooth.
Mix in the soured cream and adjust the sweetness with extra icing sugar if needed.
Layer the cakes with frosting between each sponge and over the top.
Decorate the cake with the peanut brittle shards.