These cakes are truly a treat for the senses. The rich chocolaty scent, the decadent taste and, of course, the satisfying surprise of seeing the river of warm chocolate pour out when you cut into them.
Preheat the oven to 450°F.2.
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, yoke and sugar with a whisk or electric beater until light and thick.3.
Beat 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.4.
Butter and lightly flour four 4-ounce molds, custard cups or ramekins. Tap out the excess flour, then butter and flour them again. 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 temperate before baking).5.
Bake the molds on a tray for 6-7 minutes; the center will still be quite soft, but the sides will set.6.
Invert each mold onto a plate and let sit for about 10 seconds. Unmold by carefully lifting up one corner of the mold; the cake will fall out onto the plate. Serve immediately.