Preheat oven to 350°. Grease bottom and sides of 6 (8-oz.) ramekins with butter. Lightly coat with 3 Tbsp. sugar, shaking out excess. Place on a baking sheet.
Microwave chocolate in a large microwave-safe bowl at HIGH 1 minute to 1 minute and 15 seconds or until melted, stirring at 30-second intervals. Stir in 4 egg yolks, 1/3 cup sugar, and next 3 ingredients. (Discard remaining egg yolk.)
Beat 5 egg whites and salt at high speed with a heavy-duty electric stand mixer until foamy. Gradually add 2 Tbsp. sugar, beating until stiff peaks form. Fold egg white mixture into chocolate mixture, one-third at a time. Spoon into prepared ramekins. Run tip of thumb around edges of ramekins, wiping clean and creating a shallow indentation around edges of mixture. (This will help the soufflés rise.)
Bake at 350° for 20 to 24 minutes or until soufflés rise and are set. (A long wooden pick inserted in centers will have a few moist crumbs.) Dust with powdered sugar; serve immediately with Whipped Sour Cream.
Note: We tested with Ghirardelli 60% Cacao Bittersweet Chocolate Baking Bar. Soufflés can be assembled through Step 3, and frozen up to 1 week. Bake frozen soufflés as directed.