Grease 2 (10-oz.) ramekins with butter; sprinkle with sugar. Wipe top 1/4 inch of ramekins to provide surface for soufflés to cling to.
Place chocolate and Kahlúa in heatproof medium bowl. Bring 1 inch water to a boil in medium saucepan. Remove from heat. Place bowl with chocolate mixture over water (bowl should not touch water). Let stand until chocolate is melted, stirring occasionally.
In medium bowl, whisk together egg yolks and 1 tablespoon of the sugar until mixture thickens and turns light yellow. In large bowl, beat egg whites and remaining 1 tablespoon sugar at medium-high speed until stiff but not dry peaks form.
Slowly stir chocolate mixture into egg yolk mixture. Stir in one-third of the egg white mixture. Gently fold in remaining egg white mixture. Spoon mixture into ramekins; place on baking sheet. (Soufflés can be made to this point up to 3 hours ahead. Cover and refrigerate.)
Bake 13 to 16 minutes or until soufflés have risen and sides feel slightly firm. Sprinkle with powdered sugar. Serve immediately.