Preheat oven to 160°C Fan
Grease a 23cm tin with melted butter and dust with cocoa powder
In a large saucepan, combine the Guinness and butter over medium-low heat until butter melts
Remove from heat and add cocoa powder and caster sugar, whisk until well blended
Combine all cake ingredients together and pour into prepared tin
Bake for 55 minutes to 1 hour, or until risen and firm with a clean skewer test
Let the cake cool completely in the pan on a wire rack
Add all topping ingredients into a stand mixer bowl and beat with beater attachment until thick and creamy
Once cake is fully cooled, remove from pan and place on serving plate
Spread frosting over the top of the cake only to resemble the creamy head of a pint of Guinness
