Preheat oven to 375 degrees. Butter six 6-ounce (3/4-cup) soufflé' dishes or custard cups. Stir butter and chocolate in medium saucepan over low heat until melted and smooth. Cool to room temperature by placing saucepan of chocolate into refrigerator while mixing remaining ingredients.
Using electric mixer, beat eggs and sugar in large bowl about 5 minutes. Add flour; beat until just blended. Stir in cooled chocolate. Pour into prepared dishes.
Place dishes on baking sheet. Bake until cakes are firm around edges but still move slightly in center when shaken, about 12 minutes (do not over bake, consistency in center of cakes should be pudding type texture). Cool 5 minutes, then invert onto serving plates. Server warm with vanilla ice cream.