Heat the oven to 180C/160C fan/gas4. Line a 20 x 30cm baking tray with baking parchment (leave some overhanging to help you lift the cake out later).

For the sponge, put the butter and sugars in a pan with 250ml water and heat gently until the butter has melted. Take off the heat and stir in the flour, cocoa, baking powder, eggs and soured cream to make a smooth batter. Pour into the tin and cook for 30-35 mins or until cooked through and firm to the touch. Leave to cool in the tin.

For the frosting, melt the butter and 2 tbsp cocoa together, then add the soured cream and chocolate. Add the icing sugar and stir until smooth, then leave to cool completely.

Don’t worry if the butter separates; it’ll come back when you beat it. Once cold, beat using electric beaters until the frosting is paler and fluffier. Spread over the cake and dust with cocoa (I do this while it is still in the tin). Will keep for three days in an airtight container.