Step 1

Position the rack in the middle of the oven and preheat 450 degrees.

Step 2

Generously butter and dust two 4-ounce molds, custard cups or ramekins with cocoa powder, tapping out the excess.

Step 3

Combine the butter and chocolate in a heatproof bowl set over a pan with a few inches of simmering water, ensuring the bottom of the bowl doesn’t touch the water, and stir until the chocolate is almost completely melted. (Alternately, you may heat them together in a microwave, starting with 1 minute on HIGH, then stirring, and finishing in short bursts on 50 percent power as needed until almost melted. Stir together until thoroughly combined.) Remove the bowl from the pan.

Step 4

While the chocolate and butter are melting, in a medium bowl, using a whisk or handheld mixer on medium-high, beat together the egg, yolk and sugar until light, thick and pale yellow.

Step 5

Finish stirring together the chocolate and butter; it should be quite warm. Gradually whisk the chocolate mixture into the egg mixture, then quickly beat in the flour by hand, just until combined.

Step 6

Divide the batter between the molds. (At this point you can refrigerate the desserts for up to several hours. Bring them back to room temperature before baking.)

Step 7

Transfer the molds to a baking sheet to make them easy to move in and out of the oven (and contain any possible overflow). Bake for 7 to 10 minutes — the longer you bake, the less "sauce" there will be. The center will still be quite soft and slightly dipped, but the sides will be set. Let rest for about a minute.

Step 8

Place an overturned plate on top of the mold and, holding onto the mold in one hand with a potholder, mitt or dish towel, invert the plate and the mold. (If you have a lightweight pot rest or trivet, you can set the ramekin on top of that and use it to help you invert as well.) Let sit for about 10 seconds. Unmold by lifting up one side of the mold; the cake will fall out onto the plate. It's okay if you have to tap or gently shake the ramekin to nudge it out. Serve immediately.

Step 9

VARIATIONS: Add up to 1 tablespoon of your favorite liqueur, whisked in after the chocolate and egg mixtures have been combined. For an orange-flavored cake, rub the zest of 1 orange into the sugar before combining with the eggs. To make this recipe gluten-free, you may substitute a gluten-free flour blend.

Step 10

If you are concerned about undercooked batter, you may use pasteurized eggs. Or you may choose to add ganache and fully cook the dessert. To make the ganache, in a small saucepan over medium heat, add 1/4 cup cream and bring to a simmer. In a small bowl, add 2 ounces of chocolate. Pour the hot cream over the chocolate and stir until smooth. Place ganache in the freezer until firm, 30 to 45 minutes. Fill the ramekin halfway with batter, add 1 tablespoon of the ganache and then cover with the rest of the batter. Bake for 10 to 12 minutes.