Prepare the tins: grease and base-line 2 x 20cm (4cm deep) tins with baking parchment. Grease and flour the sides of the tins too.
Make the cake batter: Sift flour, baking powder and bicarb together and set aside.
Stir stout into the cocoa, mix well and set aside.
Cream butter and sugar until pale and fluffy (10 mins with electric hand mixer). Add eggs one at a time, beating well after each addition
Fold dry ingredients (flour, baking powder and bicarb) in to the egg and butter mixture alternately with the stout and cocoa liquid.
Divide the mixture equally between the 2 tins and bake in the centre of a 170 C / 150 C fan / Gas 3 for 30 mins. When done, the cakes will have shrunk slightly away from the sides of the tins and leave no trace of batter when a wire cake tester is used.
Cool in the tins for 5 mins and turn out on to a wire rack, removing the paper bases carefully. Re-invert the cakes so that the top is uppermost for presentation and to prevent the cake tops sticking on to the racks.
Make the frosting: Beat the softened butter and icing sugar until pale and fluffy.
Add the stout and blend in well.
Melt the chocolate and fold gently into the icing mixture.
Compose the cake: Place a smattering of frosting onto your serving plate (to hold the cake in place) and sit one cooled cake layer. Frost and place second cake on top. Cover top and sides with more frosting and decorate as desired.