Preheat oven to 400° F. Butter an 8-inch springform pan and line the bottom with parchment paper or wax paper. In the top of a double boiler, melt the chocolate over simmering water. Stir in the butter. Remove from heat when smooth.

In a small bowl, beat the egg yolks until thick and pale. Stir in the flour and add the egg yolk mixture to the chocolate mixture and stir until combined.

In a separate bowl beat the egg whites until medium peeks form. Gently fold the egg whites into the chocolate mixture in three batches. Do not stir or the egg whites will lose volume.

Pour the batter into the prepared pan and bake for 18 to 20 minutes, or until the center is set but not dry. Let cool for at least 2 hours. Dust with confectioners sugar just before serving.