Cakes3 eggs85 gm caster sugar50 gm (1/3 cup) cornflour25 gm (¼ cup) Dutch-process cocoa, plus extra for dusting1 tsp cream of tartar½ tsp bicarbonate of soda1 tbsp plain flour40 gm unsalted butter, melted½ cupthickened cream¼ cup sweetened chestnut purée (available from delicatessens)Chocolate sauce200 gm dark chocolate, chopped50 gm unsalted butter
Using an electric mixer, whisk eggs and sugar until mixture holds a trail. Sift cornflour, cocoa, cream of tartar, bicarbonate of soda and flour over egg mixture and, using a spatula, gently fold in, then add melted butter and fold in until combined. Spoon mixture into two 18x26cm baking paper-lined swiss roll pans and bake at 180C for 12 minutes or until cakes spring back when lightly touched. Stand cakes for 3 minutes before turning out onto wire racks to cool.
Using an electric mixer, whisk cream until soft peaks form, then stir in chestnut purée until combined. Using a 7cm-round pastry cutter, cut 12 rounds from sponge, then spread 6 rounds with chestnut cream, top with remaining rounds and refrigerate for 5 minutes or until firm.
For chocolate sauce, melt chocolate in a bowl over a saucepan of simmering water, add butter and stir until smooth.
Serve cakes, dusted with cocoa and drizzled with chocolate sauce. Cakes will keep, refrigerated, in an airtight container for up to 3 days.