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.