Preheat the oven to 180°C/160°C Fan/Gas 4. Lightly grease a 33 x 23cm (13 x 9in) Swiss roll tin and line with non-stick baking paper, pushing it right into the corners.

Place the chocolate pieces in a bowl set over a pan of simmering water, taking care that the bowl does not touch the water. Stir until melted.

Measure the caster sugar and egg yolks into a bowl and whisk with an electric hand whisk until light and creamy. Add the cooled chocolate and stir until evenly blended.

In a separate large mixing bowl, whisk the egg whites until they look like cloud. Stir a large spoonful of the egg whites into the chocolate mixture to loosen it slightly, then fold in the remaining egg whites. Finally, fold in the cocoa powder.

Turn the mixture into the prepared tin and gently level the surface. Bake in the preheated oven for about 30 minutes until risen and firm to the touch. Remove from the oven and leave to cool in the tin (expect it to sink a little).

Meanwhile, whip the cream until it just holds its shape.

Dust a large piece of non-stick baking paper with icing sugar.

Turn the roulade out on to the baking paper and peel off the base lining paper. Score a 3cm (1¼in) line along one short edge, making sure not to cut all the way through. Spread the surface of the roulade with the whipped cream, then roll it up like a Swiss roll, starting with the scored short edge. Roll tightly to start with and use the paper to help. It will crack! But, don’t worry, it is quite normal and part of the charm.

Transfer the roulade to a long plate or board and dust with more icing sugar to serve.

Mary’s Tips: Can be made and rolled up to 24 hours ahead. Freezes well when filled and rolled.