1.
Preheat oven to 180°C. Grease and line the base and sides of a 20cm round springform cake pan. Place butter and Guinness in a saucepan over low heat and add sifted cocoa. Cook, stirring until smooth and melted. Set aside to cool for 5-10 minutes.
2.
Beat eggs, creme fraiche and vanilla in a bowl with a wooden spoon and add the flour and baking powder, followed by sugar. Spoon into prepared pan, smoothing with a palette knife, then bake for 1 hour and 15 minutes or until a skewer inserted in centre comes out clean. Cool in pan for 15 minutes, then turn out and cool completely on a rack.
3.
For the topping, combine creme fraiche and mascarpone in a bowl. Spread onto the cooled cake when ready to serve.
