Preheat the oven to 190C/170C fan/gas 5.

Grease two 20cm- or 23cm-diameter cake tins and set aside.

Cream together 225g softened unsalted butter and 225g soft dark-brown sugar until fluffy. Beat well to break down the brown sugar.

Mix in 4 large beaten eggs a little at a time. If the mixture starts to curdle, add a little of the flour, a tablespoon at a time. Don’t worry if this does happen – the cake will be fine.

Fold in the rest of the 275g sifted self-raising flour along with 4 tbsp cocoa powder, 1 tsp baking powder and a good pinch of salt, pouring in 125ml Guinness at the same time. Don’t overbeat or you’ll knock out the air.

Spoon the mixture into the tins and bake for about 30 minutes, or until the cakes are coming away from the sides of the tin and a skewer inserted into the centre comes out clean. Turn out on to a wire rack and leave to cool.

To make the buttercream, beat 75g slightly softened unsalted butter with 75g icing sugar and 75g soft dark-brown sugar until fluffy. Add 2 tbsp cocoa powder and 2 tbsp Guinness (or a little more to taste) and beat again.

For the icing, melt 150g good-quality plain chocolate (broken into small pieces) with 75ml sour cream, 75ml double cream and 4 tbsp soft-dark brown sugar in a bowl set over a pan of simmering water. Stir, take off the heat and leave to cool and thicken.

Spread buttercream icing on one of the cakes, place the other cake on top and spread on sour-cream icing with a palette knife.