Preheat the oven to 180°C.
Line a 23x33cm Swiss roll tin with greaseproof paper and brush lightly with vegetable oil.
Whisk caster sugar and egg yolks until pale and thick, then stir in flour and melted chocolate.
Whisk egg whites in a separate bowl until stiff peaks form.
Gently fold the egg whites into the chocolate mixture using a large metal spoon.
Pour batter into the tin and bake for 18-20 minutes until springy.
Cool the cake in the tin on a wire rack for at least two hours.
Turn the cake onto a tea towel, remove the tin, and peel off the paper.
Mix chestnut purée, whipped cream, and mascarpone, then fold in chopped chestnuts.
Spread filling over cake leaving a 2cm border and roll tightly using the towel.
Transfer to a dish, slice, and dust with icing sugar.