Preheat oven to 325 degrees F. Combine flour, sugar, and salt in a medium bowl.
In a large mixing bowl, beat together butter and eggs until well combined. Add buttermilk and vanilla and continue mixing. Gradually add flour mixture until all of the ingredients are incorporated.
Pour the filling into the piecrust and bake for one hour. The pie should be completely set and the top a light golden brown. Allow to cool completely before serving. Dust with powdered sugar, if desired.