Preheat the oven to 450°F. Butter and lightly dust four 6-ounce ramekins with cocoa powder. Tap out the excess cocoa powder. Set the ramekins on a baking sheet.
In a double boiler, over simmering water, melt the butter with the chocolate. ½ cup (1 stick, 113 g) unsalted butter 6 ounces bittersweet chocolate
In a medium bowl, with an electric mixer (or you can use a whisk but it will take longer to whip) beat the eggs with the egg yolks, sugar and salt at high speed until thickened and pale. 2 large eggs 2 large egg yolks ¼ cup (50 g) granulated sugar ⅛ teaspoon salt
Whisk the chocolate/butter mixture until smooth. Quickly fold it into the egg mixture along with the flour. Stir to combine. 2 tablespoons all-purpose flour
Spoon the batter into the prepared ramekins and bake for 8-10 minutes, or until the sides of the cakes are firm but the centers are soft. Let the cakes cool in the ramekins for 1 minute, then cover each with an inverted dessert plate. Carefully turn each one over, let stand for 10 seconds and then unmold. Serve immediately.