Preheat the oven to 170°C/gas mark 4. Grease and line three 20cm sandwich tins.
In a free-standing mixer, cream together the butter and sugar until light and fluffy. Add the eggs one at a time, beating well between each addition and adding 1 tbsp of the flour after each egg to prevent curdling. Sift in the rest of the flour, cocoa powder and baking powder and mix well. Add the melted chocolate and beat thoroughly.
Divide the mixture evenly between the three cake tins and bake in the oven for around 25-30 minutes until risen and firm to the touch. Leave to cool in the tins for 10 minutes, then turn out on to a wire rack to cool completely.
For the topping, put the butter in the bowl of a freestanding mixer and beat until soft. Add half the icing sugar and beat until incorporated. Add the remaining icing sugar and beat until incorporated. Add the caramel and beat again.
Break the chocolate into small pieces and place in a heatproof bowl. Heat the cream to boiling point, then pour it over the chocolate. Leave for 30 seconds, then stir well to create a thick glossy ganache. Leave to cool, then whisk with a handheld electric whisk until thick with a buttercream consistency.
Put the sugar in a heavy-based saucepan over a gentle heat until melted. Continue to cook until the sugar is a rich dark colour. Add the peanuts and stir to coat, then pour the mixture on to a baking tray lined with a sheet of greaseproof paper. Leave to cool and then break into shards.
To construct the cake, crush two of the digestives into rough crumbs and stir through the caramel buttercream. Spread half the buttercream over the top of the first layer, place another layer of cake on top and repeat. Finish with the final layer of cake. Cover the entire cake in whipped chocolate ganache. Top with the remaining broken biscuits, marshmallows and shards of peanut brittle, then drizzle with the extra caramel.