Melt the chocolate (see
Grease and line a 33 x 23 cm (13 x 9 inch) Swiss roll tin. Transfer the mixture to the tin, spreading it well into the corners, and smooth the surface with a palette knife. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes until risen and set.
Sprinkle a large sheet of baking paper with caster sugar. Remove the roulade from the oven and turn it out immediately on to the sugared paper. Carefully remove the lining paper and cover the roulade with a clean tea towel. Set aside to cool.
Put the chestnut purée and icing sugar in a food processor and purée until smooth (or combine well by hand). Transfer the mixture to a bowl and stir in the brandy. Gently whisk in the cream until light and fluffy. Spread the filling over the roulade, leaving a 1 cm (½ inch) border, and roll it up from one short end to form a log. Serve dusted with sifted icing sugar.