8 Tablespoons unsalted butter (1 stick), plus more for the molds
2 teaspoons all-purpose flour, plus more for the molds
4 ounces bittersweet chocolate
2 egg yolks
¼ cup sugar
1. Heat the oven to 450 degrees F.
2. Butter and lightly flour four 4-ounce molds, custard cups or ramekins; tap out excess flour.
3. Use a double boiler set over hot (not boiling) water to heat the butter and chocolate together until the chocolate is almost completely melted. Set aside and cool for 5 minutes. Meanwhile, beat the eggs, yolks, and sugar together with a whisk until light and thick. Whisk the melted chocolate and butter together until creamy; it should be quite warm. Pour in the egg mixture, then quickly beat in the flour, just until combined.
4. Divide the batter among the molds. Put the molds on a baking sheet and bake until the center is still quite soft but the sides are set, about 6 to 7 minutes.
5. Invert each mold onto a plate and let sit for about 10 seconds. Un-mold by lifting up one corner of the mold; the cake will fall out onto the plate. Serve immediately. Optional accompaniments: powdered sugar, vanilla ice cream, berries, or whipped cream.