Position a rack in the middle of the oven and preheat to 450º F.
Butter and flour four (4 oz.) ramekins.
Slowly melt the chocolate with the butter in the microwave; whisk until smooth.
Separately beat the eggs, yolks, and sugar with an electric mixer on high speed in a medium deep bowl for about 8 minutes, until very thick and pale.
Whisk in the warm chocolate mixture then whisk in the flour just until blended.
Divide the batter evenly among the ramekins.
Arrange the ramekins on a baking sheet and bake for 11 minutes or until the cakes have risen and have a thin crust, their sides are set, and they are still slightly jiggly at the center; underbaking is better than overbaking.
Carefully invert each ramekin onto a serving plate and let sit for 20 seconds, then unmold by lifting up on the ramekin and separating the cake from the ramekin with a butter knife.
Cake should be inverted on the plate.
Dust each cake with powder sugar.