Add 8 ounces of melted chocolate and puree until very smooth. Line a 4 cup round log-shaped mold or loaf pan with plastic wrap, pressing it down until smooth. Pour in the chestnut-chocolate mixture and tamp the mold to eliminate any air bubbles. Smooth over the surface, and cover with plastic wrap. Refrigerate overnight. To Garnish: =========== Unmold log on serving plate, and carefully remove the plastic wrap. Whip the cream with 1 teaspoon of sugar and 1/2 teaspoon of vanilla until stiff. Spread the cream evenly over all sides of the log, forming a barklike ridge here and there with a spatula or decorating comb. Drizzle thin lines of chocolate down the length of the log, using a paper pastry cone or fork. Roll the 3 reserved chestnuts in cocoa and arrange on the log lengthwise. Garnish with candied violets and refrigerate until serving. This log may be refrigerated for up to one week.