Mix 1 stick butter; cream thoroughly. Add 3/4 cup sugar; cream together. Add 1 square (1 ounce) chocolate; melt and cool. Add to butter and sugar mixture 1 teaspoon vanilla and 2 whole eggs, one at a time. Beat 5 minutes after each egg. Pour into baked shell. Let stand for several hours in refrigerator. Top with Dream Whip and pecans.