Dissolve dry milk in brandy in a small bowl; set aside. Combine chestnut puree, sugar and vanilla in a heavy medium saucepan; heat over medium-high heat, whisking until smooth. Cook the mixture, whisking constantly, until thickened and stiff, 6 to 10 minutes. (Reduce heat if the mixture begins to scorch.) Whisk in the brandy-milk mixture and continue cooking until the mixture is thick enough to stand in peaks, about 5 minutes longer. Transfer to a bowl and let cool to lukewarm, about 20 minutes.
Line a baking sheet with parchment or wax paper. Form the chestnut mixture into 1-inch balls and place on the prepared baking sheet. Cover with plastic wrap and refrigerate until chilled, about 20 minutes.
Heat 4 ounces chocolate in the top of a double boiler over hot, not boiling, water. Stir often, until the chocolate melts and reaches a temperature of 120°F. Remove the top of the double boiler from the heat. Add the remaining 2 ounces chocolate and stir until the chocolate is smooth and shiny.
Place the chilled balls of chestnut filling, close together but not touching, on a large sheet of plastic wrap. Pour all of the melted chocolate over the balls (scrape the bowl well), then roll each ball in chocolate to coat evenly. Transfer the coated truffles back to the baking sheet. (There will be about 2 ounces of chocolate left over; reserve it for another use.) Let the truffles stand at room temperature until the chocolate has set, about 5 minutes.
Make Ahead Tip: Refrigerate in an airtight container for up to 1 week or freeze for up to 6 months.
Chestnuts, unlike all other nuts, contain very little fat. Chestnut puree makes a deceptively rich-tasting filling for these elegant truffles. Glittery gold food-color “paint” gives the truffles a dramatic look. It and the chestnut puree can be ordered by mail from: La Cuisine, 323 Cameron Street, Alexandria, VA 22314, (800) 521-1176.
Storage smarts: For long-term freezer storage, wrap your food in a layer of plastic wrap followed by a layer of foil. The plastic will help prevent freezer burn while the foil will help keep off-odors from seeping into the food.