Preheat the oven to 180˚C (fan 160°C, gas mark 6). Grease and line a swiss roll tin with baking paper.

 Whisk the caster sugar and eggs in a large bowl until pale and light.

Sift together the flour and cocoa powder and gently fold into the egg mixture using a metal spoon. Pour into the prepared tin, tilting it so that it covers the whole surface.

Bake for 8-10 minutes until firm yet springy to the touch.

Dust a large sheet of paper with caster sugar and then turn the sponge onto it. Trim off the edges. Make a dent 1cm from the edge and tightly roll up the sponge and leave to cool.

Mix together the icing sugar and butter, then add the melted chocolate and milk.Beat until smooth.

Unroll the sponge, spread over a generous layer of buttercream and roll up tightly again.

Cover with the remaining buttercream, using a fork to create a log effect. For a finishing touch, dust with icing sugar.