Cook the chestnuts in boiling water until they are tender, then remove the shells and skins.
Sieve the chestnuts to make a puree.
Cream together the butter and sugar, then beat in the chestnut puree.
Melt the chocolate gently in a heatproof basin over a pan of hot water.
Add to the chestnut mixture and blend well.
Turn into a lightly oiled and bottom-lined tin.
Keep in a cool place until ready to serve.