PREHEAT oven to 425° F. Generously butter six 6-ounce ramekins or custard cups with two tablespoons butter.
STIR 3/4 cup butter and chocolate in medium, heavy-duty saucepan over low heat until chocolate is melted and mixture is smooth. Remove from heat. Beat eggs, egg yolks, sugar and vanilla extract in large mixer bowl until thick and pale yellow, about 8 minutes. Fold 1/3 of chocolate mixture into egg mixture. Fold in remaining chocolate mixture and flour until well blended. Divide batter evenly among prepared ramekins. Place on baking sheet.
BAKE for 12 to 13 minutes or until sides are set and 1-inch centers move slightly when shaken. Remove from oven to wire rack.
TO SERVE, run a thin knife around top edge of cakes to loosen slightly; carefully invert onto serving plates. Lift ramekins off of cakes. Sprinkle with powdered sugar. Serve immediately.