Pre heat the oven to 180 degrees C.
Place Guinness, cocoa, and butter into a saucepan and heat on a medium heat until the butter melts.
Place the sugar, eggs, and buttermilk into a bowl and beat with and electric mixer until combined. Pour the Guinness mix into the buttermilk mixture and stir until incorporated. Sift in the flour and baking soda and whisk until combined. Then, pour into a lined 20cm round cake tin and bake for 45 minutes or until cooked.