Preheat oven to Gas Mark 5/375F/190 °C. Grease and line a 13 in x 9 in Swiss roll tin with greased silicone paper.

Sift the flour and cocoa onto plate; set aside in warm place. Melt butter.

Using an electric mixer, whisk the eggs and caster sugar in a heatproof bowl until well-blended.

Place the heatproof bowl over large pan of hot water. Continue whisking until egg mixture is pale and leaves a trail when the whisk is lifted.

Re-sift the flour/cocoa mixture. Using a metal spoon, fold into egg.

Fold in melted butter. Fold in 1 tbsp hot water. Pour the mixture into a lined Swiss roll tin.

Bake for 10 minutes until the cake feels firm when pressed gently.

Place sheet of greaseroof paper on a damp tea-towel. Sprinkle paper with caster sugar.

Turn the baked cake straight from the oven onto the sugared paper. Remove the lining paper from the cake. Trim off cake's crusty edges.

Roll up the cake from a short side. Cover with the damp tea-towel and set aside to cool.

Uncover and unroll the cake. Spread with chestnut puree. Roll up the cake carefully over puree.

Place on serving dish. Spread with chocolate icing. Lightly dust with sifted icing sugar.