Preheat oven to 180°C conventional
Mix together plain flour, baking soda, baking powder, salt, and cocoa powder
In a separate bowl, combine white sugar and soft dark brown sugar
Add eggs and vanilla extract to the sugar mixture
Stir in vegetable oil, creme fraiche or sour cream, Guinness, and boiling water
Combine wet and dry ingredients
Pour batter into a 20cm x 30cm pan
Bake at 180°C for 35 minutes
Allow cake to cool completely
For frosting, cream together butter and cream cheese
Beat in icing sugar and vanilla extract
Spread frosting over cooled cake
