Melt the chocolate in a heatproof bowl using 30-second blasts in the microwave until just melted. Set aside to cool to room temp.
Separate the eggs into two bowls, then whisk the egg whites with an electric mixer on high speed until they stand in stiff peaks. If you turn the bowl upside down, the whites should be stiff enough not to fall out.
In the second bowl, add the sugar to the egg yolks and whisk using the electric mixer (no need to wash ion high speed for 2 to 3 minutes or until very thick and pale in colour, and the mixture leaves a ribbon-like trail on itself when the whisk is lifted out.
Pour the chocolate into the yolk mixture and fold in to blend
Add 2 large spoonfuls of the whisked egg whites and stir in gently to loosen.
Fold in the remaining egg whites using a large metal spoon. Avoid knocking out the air you have just whisked in
Sift the cocoa powder over the mixture and fold it in
Grease and line the base and sides of a 23cm x 33cm non-stick swiss roll with non-stick baking paper. If you make a small diagonal snip in each corner of the paper, it will help fit snugly into the corners of the tin.
Pour the mixture into the tin and move it around so the mixture finds its own level
Bake for 20-25 minutes or until risen. Remove and cool in the tin; the sponge will fall a little as it cools
Lay a large piece of non-stick baking paper on the worktop and dust it lightly with icing sugar. Turn the sponge out onto the paper, then carefully peel off the lining paper
Whip the cream for the filling until just holds its shape, then spread it over the sponge, leaving a clear edge of about 2cm on all sides
Using a sharp knife, make a shallow cut along one of the short edges
Roll this cut edge over tightly to start. Use the sugar-dusted paper to continue tight rolling by pulling it away from you as you roll. Finish with the join underneath
Lift the roulade onto a serving plate or board using a large wide spatula or 2 fish slices. Serve in slices and enjoy!