Preheat the oven to 180degC or 160degC Fan. Butter and line the tin (7 inch spring form or 8 inch Bundt cake tin).
Pour the Guinness into a wide saucepan, add the butter (in spoons or slices) and heat until the butter is melted, at which time you whisk in the cocoa and sugar. Beat the sour cream with the egg and vanilla, and pour into the brown and beery pan. Lastly whisk in the flour and bicarbonate.
Pour the cake batter into the tin and bake for 45-60 minutes. Leave to cool completely in the tin on a cooling rack. It is quite a damp cake.
When the cake is cold, sit it on a flat platter or cake stand. Sprinkle the top with icing sugar.
