Heat the oven to 325 degrees. Butter an 8-inch springform pan, and set aside. Place both chocolates in a large heat-proof bowl, and set over a pan of simmering water. When chocolate is completely melted, add sugar and butter, and stir until well combined. Remove from the heat, and stir in egg yolks.
Place egg whites in the bowl of an electric mixer fitted with the whisk attachment; whip until frothy. Sprinkle flour over whites; beat on medium until stiff but not dry.
Add 1/3 of the egg-white mixture to the chocolate mixture, and stir until thoroughly combined. Very gently fold remaining whites into the chocolate mixture until well blended.
Pour the batter into the prepared pan, and bake until springy but still soft in the middle, about 40 minutes. Cool on a rack for 1 hour. Cover with plastic wrap, and chill overnight. Remove from refrigerator, and unmold about 1 hour before serving.