Butter bottom and sides of 8 (6-ounce) ramekins; sprinkle with sugar. Set aside.
Melt 1/4 cup butter and chocolate in a saucepan over medium-low heat. Add flour; stir until smooth. Cook 1 minute, stirring constantly. Gradually add chocolate milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and vanilla. Beat egg yolks and sugar at medium speed with an electric mixer until thick and pale (about 2 minutes). Gradually stir about one-fourth of hot chocolate mixture into yolk mixture; beat at medium speed until blended. Gradually add remaining hot mixture, beating until blended. Let cool 5 minutes.
Beat egg whites and cream of tartar at high speed until stiff peaks form. Gently fold one-fourth of beaten egg white into chocolate mixture. Gradually fold remaining egg white into chocolate mixture. Carefully spoon mixture into prepared ramekins. Cover and freeze until firm. (See Note below to bake them immediately.)
Remove soufflés from freezer, and let stand 30 minutes. Bake at 350° for 30 minutes or until tops are puffed. Cut a slit in top of each soufflé, and serve immediately with Irish Cream Sauce.
Note: Find dark chocolate bars on the candy aisle at your local grocery store. Other types of chocolate used in this chapter can be found on the baking aisle.
Note: You can bake these soufflés immediately without freezing. Bake at 350° for 30 minutes.