3/4 c. butter, softened (no substitute)
1 c. sugar
1 1/2 tsp. vanilla
1/4 tsp. cream of tartar
3 (1 oz.) sqs. unsweetened chocolate, melted and cooled
3 eggs
9 inch graham cracker crust
Whip cream (optional for top)

In large mixer bowl, beat butter until light and fluffy. Gradually sprinkle in sugar. With mixer at high speed, beat in vanilla, tartar, and chocolate. Add eggs, one at time, beating 4 minutes after each addition and scraping bowl. Pour into crust. Cover and refrigerate 3 to 4 hours until set.