In top of double boiler, melt chocolate and butter over simmering water. In large bowl, combine sugar, yolks, Kahlua and coffee powder. Blend in chocolate mixture. Whip cream until stiff peaks form; fold into Kahlua-chocolate mixture. Beat egg whites until soft peaks form; fold into mixture. Spoon into dessert glasses. Refrigerate 4 hours or overnight. Top, if desired, with whipped cream, chocolate curls and mint leaves. (6 to 10 servings)