Few desserts elicit the oohs and ahhs that molten cakes do, which makes them a natural pick for special dinners. Feel free to use semi-sweet baking chocolate in place of the bittersweet chocolate if that's the variety you have on hand.
Preheat oven to 425 degrees F. Using 2 tablespoons butter, grease six 8- to 10-ounce ramekins, souffle dishes, or custard cups. Place ramekins in a 15x10x1-inch baking pan; set aside.
In a heavy small saucepan, combine chocolate and 3/4 cup butter. Cook and stir over low heat until chocolate melts. Remove pan from heat; set aside.
In a large mixing bowl, beat eggs, egg yolks, granulated sugar, and vanilla with an electric mixer on high speed for 8 to 10 minutes or until thick and lemon color. Fold one-third of the chocolate mixture into egg mixture. Fold remaining chocolate mixture and flour into egg mixture. Spoon about 2/3 cup batter into each prepared ramekin.
Bake about 12 minutes or until cake edges feel firm. Cool in ramekins on a wire rack for 2 to 3 minutes. Using a knife, loosen cakes from sides of ramekins. Invert onto dessert plates. Sift with powdered sugar and cocoa powder. Serve immediately.