In a large bowl, cream butter until light and fluffy. Gradually add sugar, and beat well. Beat in the eggs. Add flour, buttermilk, and vanilla; beat until smooth
Pour filling into pie shell, and bake at 400 degrees F (205 degrees C) for 5 minutes. Reduce heat to 350 degrees F (175 degrees C); bake for an additional 45 minutes, or until set.
Cool at room temperature, then chill. When serving, drizzle individual servings of pie with molasses for that unbeatable combination of flavors!