Preheat oven to 350ºF. In a 4 quart saucepan, melt the butter, chocolate, and sugar over low heat. Cool. Grease ten 6 ounce ramekins, dust with flour. In a small bowl, with mixer at high speed, beat the egg yolks for 3 minutes or until thick. In a large bowl, with mixer at high speed, beat the egg whites until stiff. Whisk yolks into chocolate mixture, fold in whites and flour. Place ramekins in roasting pan. Spoon the batter into ramekins. Pour boiling water into pan to come halfway up sides of ramekins. Bake for 40 minutes or until the knife inserted near the center comes out clean. Invert to serve.