Step 1

Preheat the oven to 450°F and set an oven rack in the middle position. Generously butter six 6-ounce ramekins (really be generous, and do not substitute non-stick cooking spray). Set the ramekins on a baking sheet.

Step 2

Melt the butter in a medium bowl in the microwave. Immediately add the chopped chocolate and stir until the chocolate is completely smooth and melted. If a few chunks remain, place the bowl back in the microwave for 20 seconds, then stir again. Repeat if necessary. Set aside.

Step 3

In the bowl of an electric mixer fitted with the paddle or beaters, beat the eggs, egg yolks, brown sugar, vanilla and salt at high speed until thickened and pale, about 4 minutes. Add the melted chocolate mixture and flour to the egg mixture and fold with a rubber spatula until just combined (the chocolate sinks to the bottom so be sure to scrape it up into the batter).

Step 4

Spoon the batter evenly into the prepared ramekins and bake for 10 minutes, until the cakes are set and puffed over the edges of the ramekins. Let the cakes cool in the ramekins for 1 to 2 minutes; then, holding the hot ramekins with a dish towel, carefully invert the cakes onto dessert plates. They should pop right out, but you can run a thin-bladed knife around the edges if necessary. Serve immediately. (It's important not to let the cakes sit after baking, as the centers will continue to cook.)

Step 5

Make-Ahead Instructions: The batter can be spooned into the ramekins, covered and refrigerated for several hours before baking. Take them out of the fridge about 30 minutes before baking so they come to room temperature.