1. In a medium-sized saucepan, warm half of the milk with the sugar, salt, and cocoa powder.
2. Bring to a full boil while whisking, then reduce the heat and simmer gently for 30 seconds.
3. Remove from heat and add the chocolate, the vanilla, and the coffee-flavored liqueur, if using. Stir in the other half of the milk.
4. Taste, and if the chocolate is a bit grainy, puree it in a blender to smooth it out.
5. Chill thoroughly, then freeze in your ice cream maker according to the manufacturer’s instructions.