Grease and line a 20.5 x 30.5 cm (8 x 12 inch) Swiss roll tin. Mix the cocoa powder and milk in a small saucepan and heat gently until the cocoa powder has dissolved. Remove the pan from the heat and set aside to cool.

Whisk the egg yolks and sugar together until pale and fluffy. Whisk the cooled milk mixture into the egg yolk mixture.

Whisk the egg whites until stiff, then fold into the cocoa mixture. Spread the mixture evenly into the prepared tin and bake at 180°C (350°F) mark 4 for about 20 minutes until the sponge has risen and is just firm to the touch.

Turn out on to a sheet of greaseproof paper and cover with a warm, damp tea-towel to prevent the sponge from drying out. Leave the sponge to cool for 20 minutes.

Meanwhile, whip the cream until stiff. Spread over the sponge, reserving half for decorating and then roll it up carefully. Do not roll it up too tightly and do not worry if it cracks slightly. Pipe the reserved cream on top and decorate with strawberries and grated chocolate. Serve chilled.