1/2 cup unsalted butter, plus more to butter the molds
4 ounces bittersweet chocolate
2 eggs
2 egg yolks
1/4 cup sugar
2 teaspoons flour, plus more for dusting
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 the butter & chocolate are heating, beat together the eggs, yolks, and sugar with a whisk or electric beater until light and thick.
Beat together the melted chocolate and butter; 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 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 temperature before baking.)
Preheat the oven to 450 degrees. 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. Serve immediately.