Americans often bake cakes with a batter mixture using oil instead of a solid fat, to produce a light texture.
Make the chocolate icing while the cake is cooling. Sift 300g (10oz) icing sugar and 25g (1oz) cocoa powder in a bowl. Melt 125g (4oz) butter with 4tablespoons water, then beat into the icing sugar until just runny. The icing does crack once set, if moved, so ice the cake on the serving plate with pieces of grease proof paper under the cake.
Preheat the oven to 160°C, 325°F, Gas Mark 3. Grease and line a 25cm (l0in) square cake tin.
Melt 125g (4oz) dark chocolate with 125g (4oz) soft brown sugar in 300ml (½pint) milk, then stir in 150ml (¼pint) sunflower oil and 1teaspoon vanilla essence until well blended. Sift 350g (11oz) plain flour, 2tablespoons cocoa powder, 1teaspoon bicarbonate of soda and apinch of salt into a large mixing bowl.
Beat in the liquid ingredients, 250g (8oz) caster sugar and 4 eggs. Pour into the tin and bake for 1 hour or until a skewer comes out clean. Cool for 5 minutes, then turn out and remove the paper. Put on a wire rack and slice in half. Cool completely.
Meanwhile, make the icing (above). Use some icing to sandwich the cake halves together, then pour the rest immediately over the top and down the sides. Decorate with 12 walnut halves and allow to set. Cut into 12 pieces to serve.