Position rack in middle of oven and preheat to 450 degrees F. Butter and flour four 4-ounce ramekins.

Melt chocolate with butter. Remove from heat. Beat eggs, yolks and sugar with an electric mixer on high speed in a deep bowl, 8 minutes, until thick and pale. Whisk chocolate mixture into egg mixture. Whisk in flour.

Pour batter into ramekins. Place on baking sheet and bake 14 minutes, or until cakes have risen and have a thin crust, sides are set and cakes are slightly jiggly in the center; under baking is better than over baking.

Carefully invert each cake onto a serving plate and let sit 20 seconds. Unmold by lifting up one corner of mold; cake will fall onto plate. Dust tops with confectioners' sugar and serve immediately.