This souffle is simple to make and won't fall as easily as some do.
Heat oven to 450 degrees. Butter a 4-cup souffle dish and coat with granulated sugar. If desired, attach a parchment-paper collar, securing it with kitchen string. Set aside.
Combine chocolate and butter in a small heat-proof bowl, and set over a saucepan of barely simmering water until almost melted. Remove from heat and stir until completely melted. Stir in liqueur or coffee and egg yolks.
Beat egg whites with cream of tartar until stiff peaks form; don't overbeat. Whisk a quarter of the egg whites into chocolate mixture. Fold in remainder with a rubber spatula. Pour into prepared souffle dish.
Bake on bottom rack for 5 minutes. Lower temperature to 425 degrees and continue baking, 10 minutes for a moist center, 15 minutes for a dry center. Dust with confectioners' sugar and serve.