5 issues of BBC Good Food for £5
Heat oven to 200C/180C fan/gas 6. Grease a 25cm springform tin, then line the base and sides with baking paper. Whisk eggs and sugar until pale and thick.
Sift the flour and cocoa together onto the mixture, then gently fold in, followed by the butter. Pour into the tin, ease to the edges, then bake for 8-10 mins until risen and firm to the touch. Cool in the tin.
Meanwhile, beat the chestnut purée and egg yolks until as smooth as possible. Melt the chocolate in a bowl over a pan of simmering water with half the cream. Remove from the heat, then beat into the chestnut mixture.
Whisk the egg whites until stiff. In a separate bowl, whip the remaining cream until it holds its shape. Fold into the chocolate mixture, then carefully fold in the egg whites. Drizzle the brandy, if using, over the sponge base, then pour the chocolate mix on top. Level the surface and chill for 5 hrs or overnight until firm. Chill for 2 days, or freeze for 2 months. Thaw overnight in the fridge.
To serve, carefully remove from the tin, strip off the paper and slide onto a cake stand. Dust heavily with cocoa.