Use a double boiler to heat the butter and chocolate together until the chocolate is almost completely melted. While that's heating, beat the eggs, yolks, and sugar together with a whisk or electric beater until light and thick.

Beat the melted chocolate and butter together ; it should be quite warm. Pour in the egg mixture, then quickly beat in the flour, just until combined.

Butter and lightly flour four 4 ounce molds, custard cups, or ramekins. Tap out excess flour. Divide the batter among the molds. ( At this point you can refrigerate the desserts until you are ready to eat, for up to several hours; bring them back to room temperature before cooking.)

Preheat the oven to 450 degrees F. Bake the molds on a tray for 6 to 7 minutes. The center will still be quite soft, but
the sides will be set. Invert each mold onto a plate and let sit for about 10 seconds. Unmold by lifting up one corner of the mold. The cake will fall out onto the plate.