Preheat the oven to 170C then line the base of the cake tin with greaseproof paper. Carefully pour the guinness into a saucepan along with butter and gently heat until it has sufficiently melted. Take the saucepan off the heat and add the cocoa powder and the sugar into the mixture. In a separate jug or bowl mix together the eggs, vanilla essence and milk by hand, then add this to the mixture in the pan. Sift together the remaining ingredients for the sponge into a large bowl and mix well until all the ingredients are combined.
Pour the cake mix into the cake tin and bake for at least 60 minutes or until the sponge bounces back when you lightly press on it, or place a skewer in the centre of the cake and if it comes out clear, it is sufficiently baked. Allow the cake to cool, then remove it from the cake tin and place on a wire rack to cool for longer. Make sure that the cake is completely cooked before you begin to ice it, preferably it should be left overnight. For the icing, break the chocolate into small pieces. Heat the milk in a small pan until boiling then add the white chocolate until it has melted. Take the pan off the heat and whisk in the sieved icing sugar. Set aside the icing to cool before spreading it across all of the cake.
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