Preheat the oven 200C/400F/Gas 6.
For the roulade, place the egg yolks and sugar into a bowl and whisk together until pale.
Fold in the flour and cocoa powder.
Place the egg whites into a separate, clean bowl and whisk until stiff peaks form when the whisk is removed. Gently fold the egg whites into the roulade mixture.
Grease and line a swiss roll tin and sprinkle a little caster sugar over the base of the tin. Pour the roulade mixture into the tin.
Transfer the roulade to the oven and bake for 12 minutes, or until risen. Remove from the oven and leave to cool slightly.
For the filling, place all of the filling ingredients into a bowl and mix well. Spread the filling over the cooled roulade.
For the chocolate sauce, heat the water in a saucepan, add the cocoa powder and icing sugar and simmer for 3-4 minutes.
To serve, place the roulade onto a large plate, drizzle over the chocolate sauce and sprinkle with the chopped chestnuts.