Preheat the oven to 180°C and grease a 25cm loose-bottomed cake tin.
Whisk eggs and caster sugar for 5 minutes until thick and pale.
Fold in sifted flour, cocoa powder, and melted butter until just combined.
Bake the mixture for 35 minutes at 180°C then cool on a wire rack.
Whisk cream, coffee, and icing sugar into soft peaks.
Beat mascarpone until smooth and fold in the coffee cream mixture.
Blend mascarpone and chestnut purée in a processor until smooth.
Whip double cream to soft peaks and fold in the chestnut mixture.
Boil sugar and water to a syrup until it reaches 110°C.
Whisk egg whites in an electric mixer until firm peaks form.
Continue heating the syrup until it reaches 121°C.
Pour the hot syrup into the egg whites while whisking at low speed.
Beat the meringue for 5 minutes until it is tepid.
Slice the cake into five horizontal layers.
Drizzle the base layer with brandy, spread coffee mousse, and scatter crushed meringue.
Layer with sponge, brandy, chestnut mousse, and more meringue.
Repeat layering until the final sponge layer is placed on top.
Coat the cake with Italian meringue and create peaks using a fork.
Brown the meringue with a blow torch and garnish with holly.