Preheat the oven to 170 C, 150 C fan, Gas mark 3. Grease and line the base of a 23cm (9 inch) spring form tin OR a deep 20cm (8 inch) tin.
Pour the Stout into a saucepan and add the Stork, gently heat until melted. Remove from heat and stir in the cocoa and sugar. Mix together the eggs, buttermilk and vanilla and then add to the pan.
Sift together remaining dry ingredients into a large bowl, whisk in liquid ingredients until smooth.
Pour cake mix into prepared tin and bake in preheated oven for 45 minutes or for the deep 20cm tin for 1 hour 10 mins or until cooked. Insert a skewer into the middle of the cake and if it comes out clean, the cake is cooked. Cool in tin and then place on wire rack until cold.
Place the icing ingredients in a bowl and mix until light and fluffy.
Spread the cake with the icing and finish with a light dusting of cocoa powder.