Add the cold cubed butter and mix on low until the butter is pea sized. Add the buttermilk to the mixture then mix on low speed until the dough is rough and shaggy.
Transfer dough to a lightly floured surface then shape into a disc. Wrap the disc in plastic wrap and refrigerate for at least one hour or up to 3 days.
Pour the filling into the chilled pie crust. Bake the pie until the center of the pie is a little wobbly. Cool on a wire rack for at least 2 hours then transfer to the fridge.