1.Preheat oven to 180°C (160°C fan-forced). Grease a 23cm x 32cm Swiss roll pan; line the base and long sides with baking paper, extending the paper 5cm over the sides.
2.Beat the egg yolks and sugar in a medium bowl with an electric mixer for about 5 minutes or until thick and creamy. Transfer the mixture to a large bowl; fold in the hot water and chocolate, then fold in the sifted flour.
3.Beat egg whites in a clean bowl with an electric mixer until soft peaks form; fold into the chocolate mixture. Spread the mixture evenly into the pan.
4.Bake sponge for about 12 minutes.
5.Meanwhile, place a piece of baking paper cut the same size as the pan on the bench; sprinkle with extra sugar.
6.Turn hot sponge onto the sugared paper; peel away the lining paper. Using the paper as a guide, loosely roll the sponge from the long side. Stand for 2 minutes; unroll. Cool; trim all sides of sponge.
7.Make vanilla cream: Beat all the ingredients in a small bowl with an electric mixer until soft peaks form.
8.Spread sponge with cream. Again, using the paper as a guide, roll the sponge from the long side. Serve sponge roll topped with the extra grated dark chocolate or small chocolate curls, if desired.
Not suitable to freeze or microwave. The filled sponge roll is best eaten on the day it is made. Keep it in an airtight container in the refrigerator until ready to serve.Note