Preheat the oven to 220°C.
Line a 23x33cm Swiss roll tin with oiled greaseproof paper.
Whisk caster sugar and egg yolks until light and thick.
Melt chocolate with 4 tbsp water over simmering water and stir in the egg mixture.
Whisk egg whites until stiff and gently fold into the chocolate batter.
Pour batter into the tin and bake for 12 to 14 minutes.
Cool the cake in the tin on a wire rack for 2 hours.
Invert the cake onto fresh paper, remove the tin, and peel off the backing.
Beat butter, icing sugar, cocoa, and milk to make buttercream.
Spread half the icing or the cream and berries over the cake.
Roll the cake tightly from the long side using the paper for support.
Cover with remaining icing or dust with icing sugar before serving.