Put chocolate into a large heatproof bowl. Bring cream just to a boil in a small saucepan over medium-high heat; pour over chocolate in bowl. Stir in liqueur, if desired. Cover with plastic wrap; let stand 10 minutes. Stir until smooth. Let stand until thick, about 15 minutes.

Pour chocolate mixture into a shallow 8-inch dish or pie plate. Cover with plastic wrap, and refrigerate until mixture is very cold and set but still pliable, about 30 minutes.

Using a teaspoon or a 1/2-inch melon baller, scoop balls of chocolate mixture, transferring them to a baking sheet lined with parchment paper as you work. Refrigerate truffles 10 minutes.

Using hands dusted with cocoa powder, dip each truffle in cocoa powder to coat, then quickly shape truffle into a rough round.