Prepare a 9x9" baking pan by lining it with parchment paper (cut the parchment in about a 12x12" square, crease the parchment then cut vertical slits in the corners to help the parchment fit into the pan).
If your chocolate is in bars or squares, chop it using a chef's knife. Set aside.
Heat 1 cup whipping cream in a medium saucepan over medium heat. Don't let it come to a boil. Once it's steaming, stir in 18 ounces high-quality dark chocolate, and continue stirring until the chocolate is completely melted and the mixture is uniform. Remove the saucepan from heat and stir in 1 Tablespoon liqueur.
Pour the chocolate mixture into the prepared baking pan. Cover the pan with plastic wrap and transfer the pan to the fridge or freezer to set (4 hours in the fridge, 1-2 hours in the freezer).
Turn the set chocolate onto a cutting board and peel the parchment off. If desired, cut off the edges to make perfect squares. Cut the squares into your desired size (I recommend 1-2 inches). Sift about 1 Tablespoon of Dutch-processed cocoa powder over the cut pieces, if desired. Keep the chocolate in an airtight container in the fridge.
Notes
Adding alcohol to nama chocolate is common and does add flavor, but it's not necessary. Feel free to skip it if you want. If you do use it, choose one that has a flavor that you enjoy, and that will complement dark chocolate. I used amaro, which is an herb-infused brandy. Mine tasted very citrusy. Rum is also a common choice, as is brandy.