Grease 6-inch souffle dish generously with butter; coat inside with sugar, shaking out excess.
Combine 2 tablespoons cold water and chocolate chips in top of double boiler.
Place over hot water until chocolate chips are melted; beat with wire whisk until blended.
Cut 2 tablespoons butter into small pieces; add to chocolate 1 piece at a time, beating until butter is melted.
Cool slightly.
Place egg yolks in large mixer bowl; beat with electric mixer until lemon-colored.
Add confectioners' sugar gradually; beat until thick.
Add 1/4 of chocolate mixture; beat with wire whisk until blended.
Add remaining chocolate mixture; beat until well mixed.
Beat egg whites and cream of tartar with electric mixer until stiff peaks form.
Fold 1/4 of egg whites into chocolate mixture; blend well.
Gently fold in remaining egg whites until well mixed.
Spoon into prepared souffle dish; smooth top.
Bake in preheated 400°F oven 35 minutes or until set.
Dust with additional confectioners' sugar, if desired; serve immediately.