In the top of a double boiler set over simmering water, heat the butter and chocolate together until the chocolate is almost completely melted. While that’s heating, beat together the eggs, yolks, and sugar with electric beater until light in color and thick. Takes 5-8 minutes.
Mix together the melted chocolate and butter; it should be quite warm. Pour in the egg mixture, and then quickly beat in the flour, just until combined.
Butter and lightly flour 4 four-ounce molds. Tap out the excess flour, then butter and flour them again.
Divide the batter among the molds. At this point you can cover and refrigerate the desserts until you are ready to cook, for up to several hours. Bring them back to room temperature before baking.
Preheat the oven to 450 degrees. Bake the molds on a tray for 6-7 minutes; the center will still be quite soft but the sides will be set.
Invert each mold onto a plate and let site for about 10 seconds. Unmold by lifting up one corner of the mold; the cake will fall out onto the plate. Serve immediately.
Best served with vanilla ice cream or sweetened whipped cream.