Cut a cross in each chestnut and roast in a hot oven (220 °C) for about 25 minutes, or until they burst. Then shell and skin.
Put the chestnuts into a pan, cover with water and add the lemon peel and cinnamon stick.
Bring to the boil, cover and simmer for about 20 minutes.
Then drain thoroughly and remove the cinnamon stick.
Roughly chop half of the chestnuts and puree the rest.
Mix the chestnut puree with the icing sugar, cocoa and ground almonds.
Heat the mixture over a low heat, stirring constantly, for about 3 minutes, then beat well.
Leave to cool slightly then stir in the egg yolk and chopped chestnuts.
Place the oblate squares on a slab and spread the chestnut mixture smoothly on top, moistening the spoon if necessary.
Leave to dry for several days.
For the icing
Beat the egg whites until stiff.
Gradually stir in the icing sugar and lemon juice to produce a spreadable icing.
Ice the cakes with the icing, then leave to dry for several hours before cutting into pieces.
Store in an air-tight container.