Fit piecrust into a 9-inch deep-dish pieplate; fold edges under, and crimp. Bake at 400° for 6 minutes; cool crust on a wire rack. Reduce oven temperature to 350°.
Meanwhile, beat butter, sugar, and flour at medium speed with an electric mixer until blended. Add eggs, buttermilk, and vanilla; beat well. Fold in chocolate morsels. Pour into prepared crust.
Bake at 350° for 1 hour until just set and lightly browned. Cool completely before serving.
Tip: Make this pie the day before you plan to serve it so it has plenty of time to cool.