Melt the bittersweet chocolate, and allow to cool slightly.
Place the butter or margarine, salt, and vanilla in a mixing bowl.Beat with a mixer until very light and airy, about 4 minutes.Add the powdered sugar a little at a time while beating on low speed.Mix well, and beat on medium speed for about 4 minutes. Add the eggs one at a time, and beat for 5 minutes more. Add the melted chocolate, and beat 4 minutes.
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