Melt chocolate in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Remove from heat and cool to room temperature.
Stir egg yolks, 1 at a time, into chocolate until evenly mixed. Pour sweetened condensed milk into chocolate mixture and stir until blended; add coffee-flavored liqueur and return double boiler to heat. Simmer until mixture is heated through, 2 to 3 minutes.
Transfer chocolate mixture to the bowl of a stand mixer; beat in butter, 1 tablespoon at a time, until fluffy. Cover bowl with plastic wrap and chill, 4 hours to overnight.
Roll mixture into 3/4-inch balls. Pour cocoa powder into a small bowl; roll balls in cocoa powder until coated. Arrange balls on a plate and refrigerate until set, at least 1 hour.