Pre-heat the oven to 150°C / fan 140°C / gas mark 2.
To make the meringues, separate the eggs (reserving the yolks) and whisk the whites until stiff. Fold in 140g of caster sugar, the cocoa powder and the icing sugar. Spread the mixture into two circles of about 25cm on two baking trays lined with parchment paper. Cook in the oven for 90 minutes. Leave to cool.
To make the chocolate mousse, melt the dark chocolate, then add 85g butter and the Tia Maria. Whisk in three egg yolks, the chestnut purée and the remaining caster sugar until pale. Whisk the double cream until stiff and fold half of it into the chocolate mix, reserving the other half. Place into the fridge.
To make the white chocolate mousse, melt the white chocolate, add rest of the butter and whisk in the remaining two egg yolks. Fold in the remaining whisked double cream and place in the fridge.
Once the meringues are cold and you’re ready to assemble the dessert, place one meringue onto a serving plate. Cover with the dark chocolate mousse mix, top with the second layer of meringue, then top with the white chocolate mousse. Dust with a small amount of cocoa and chocolate shavings. Place back in the fridge until ready to eat.