Cream the butter and sugar together in a mixing bowl until pale and creamy. Sift the flour, cocoa, baking powder and bicarbonate of soda into a bowl. Gradually beat in alternate spoonfuls of egg, flour mixture and Guinness until all have been added and the mixture is smooth.
Spoon into a 20 cm (8 inch) spring-form tin, greased and base-lined with oiled greaseproof paper, and spread the surface level. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 45–55 minutes until well risen, the top is slightly cracked and a skewer inserted into the centre comes out clean. Leave to cool in the tin for 10 minutes then loosen the edges, turn out on to a wire rack and peel off the lining paper.
Make the white chocolate frosting. Bring half the cream just to the boil in a small saucepan, then remove from the heat. Add the chocolate, set aside for 10 minutes until melted. Stir then chill for 15 minutes. Whip the remaining cream then whisk in the chocolate cream until thick. Chill for another 15 minutes.
Transfer the cake to a serving plate and spoon the chocolate cream over the top. Decorate with chocolate curls and dust with sifted cocoa powder.