1. Chop chocolate finely into1/4-inch pieces using a serrated knife; place in a large heat-proof bowl. 

2. Bring cream to a boil over medium-high heat; pour directly over chopped chocolate.  Allow to sit for 10 minutes; use a rubber spatula to stir chocolate and cream until totally combined. Add liqueur, if desired, and stir to incorporate. Allow to sit at room temperature until just thickened, about 15 minutes.

3. Pour ganache mixture into a shallow glass dish. Cover with plastic wrap, and refrigerate until mixture is very cold and set but still pliable, about 30 minutes.

4. Using a small spoon, scoop out ganache into 1-inch nuggets; place on a parchment-lined baking sheet. Chill for 10 minutes. Using fingertips to mold truffles into round shapes. Roll in cocoa powder to coat. Then place a few truffles at a time in a large sieve, and shake to get rid of the excess cocoa. Place in an airtight container until ready to serve.