Cut chestnuts crosswise and roast in preheated oven at 220°C (approximately 425°F) for about 30 minutes or until shells are browned and cracked. Remove from the oven, cool and peel. Combine cream with orange peel and slit vanilla pod, bring to a boil and add peeled chestnuts. Simmer for about 10 minutes on low heat. Strain through a sieve, taking care to collect all cream. Remove orange peel and vanilla pod, press chestnuts through a ricer. Let cool down.
Combine dark chocolate with 250 grams (approximately 9 ounces) of whole milk chocolate, chop and place into a bowl. Bring cream to a boil again, pour over chocolate and stir with a spatula until smooth. Add chestnuts liqueur. Beat butter until fluffy and stir into chocolate cream. Refrigerate, covered, for about 2 hours. Mix occasionally.
Make about 40 truffles with a teaspoon. Arrange on a plate and refrigerate for about 1 hour, covered.
Grate remaining whole milk chocolate. Shape trufflles into balls and roll in grated chocolate. Serve.