Step 1 Heat the oven to 350 degrees.
Step 2 Butter 4 (6-ounce) ramekins and dust them with granulated sugar, tapping out the excess.
Step 3Melt the butter and chocolate together in the top of a double boiler set over, but not touching, simmering water. Remove the chocolate from the heat when melted, but keep it warm.
Step 4Whip the egg yolks with 2 tablespoons of sugar at high speed until thick and pale yellow, 5 to 7 minutes. Fold the yolk mixture into the melted chocolate.
Step 5Butter 4 (6-ounce) ramekins and dust them with granulated sugar, tapping out the excess.
Step 6In a clean bowl with a clean whisk, whip the egg whites until frothy. Gradually add the remaining 2 tablespoons of sugar and whip until very soft peaks form. Stir about a third of the whites into the chocolate batter to lighten it. Then pour the remaining beaten egg whites over the top. Gently fold the egg whites and chocolate mixture together. Sift the cake flour over the batter and gently fold it in.
Step 7Pour the batter into the ramekins and bake the cakes until the tops are puffed and set and a cake tester inserted into the center comes out with moist crumbs attached, 20 to 22 minutes. Sprinkle each with powdered sugar and serve warm or at room temperature. The cakes will fall slightly as they start to cool.