Place cookie sheet in refrigerator and chill 1 hour. When chocolate is hard, trim edges with a knife and cut into six 3" squares. Place 1 square on a piece of parchment paper and create a box by standing 4 pieces around it (reserve remaining piece for lid). Dip a small metal spatula in very hot water, wipe off excess water, and using the hot spatula, heat outside seams of box so chocolate melts slightly and edges adhere to each other. (If at any time chocolate gets too soft, put it back in refrigerator until firm again.) Keep chilled until ready to use.
In a small saucepan, bring cream to a boil. Remove from heat and add chocolate, stirring gently until smooth. Add butter and pour into a small plastic bowl. Chill 3 hours.
Line a cookie sheet with parchment paper. Using a melonballer, scoop chocolate into 12 to 20 balls. Roll half the balls in cocoa and the other half in nuts. Place on cookie sheet and chill an additional hour (or put in the freezer 20 minutes).
Place truffles in box and cover with remaining piece of chocolate.
Recommended tools: A Silpat is a nonstick, reusable baking-pan liner, available at www.surlatable.com and Williams-Sonoma stores nationwide.