Preheat oven to 180C / 350F / Gas 4. Grease two 18 cm/7 in round sandwich cake tins and line the base of each with non-stick baking paper. Select a small saucepan and a heatproof bowl that will fit over it. Place the chocolate and the milk in the bowl. Bring a small saucepan of water to just blow simmering point. Place the bowl containing the chocolate mixture on top. Leave for about 5 minutes, until the chocolate softens, then stir until smooth.

Cream the butter or margarine with the sugar in a mixing bowl until pale and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the chocolate mixture until well combined.

Sift the and coca over the mixture and fold in with a metal spoon until evenly mixed. Scrape into the prepared tins, smooth level and bake for 35 -40 minutes or until well risen and firm. Turn out on to wire racks to cool.

Sandwich the cake layers together with a thick, even layer of chocolate buttercream. Dust with a mixture of icing sugar and cocoa just before serving.