Preheat oven to 450 degrees F.
Butter and lightly flour 4 (4-ounce) molds, ramekins, or custard cups. Tap out excess flour; butter and flour them again; set aside.
Place butter and chocolate in the top of a double boiler or heat-proof bowl set over a pan of simmering water, and heat until the chocolate has almost completely melted.
In the bowl of an electric mixer fitted with the paddle attachment, beat together the eggs, yolks, and sugar until light and thick.
Add melted chocolate mixture; beat to combine. Quickly beat in flour until just combined.
In another bowl of an electric mixer, whip the egg whites until soft peaks form, then fold into the mixture. Divide batter evenly among the molds.
Place filled molds on a baking sheet, and bake until the sides have set but the centers remain soft, about 6 to 7 minutes.
Invert each mold onto a plate, and let rest 10 seconds. Unmold by lifting up 1 corner of the mold; the cake will fall out onto the plate. Serve immediately.
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