In a medium bowl, whisk together flour, sugar, and salt. Cut in cold butter with a fork or pastry cutter, working until you've got a coarse, mealy crumb about the size of peas.
Make a well in the center then add about 1/4 cup of the chilled buttermilk. Blend with a fork, trying to moisten as much of the flour as possible. You want the dough to just barely come together, so add more liquid as needed, but add only about 1 tbs at a time from here.
Turn the dough out onto a lightly floured work surface then gently knead and press into a disk. Wrap in plastic wrap and chill for 1 hour. Now's a good time to start with the pie filling.
Once chilled, return the dough to a floured work surface, then roll out to a circle about 12 inches in diameter, or about 1/8 inch thick. Transfer it to the pie pan and fold, crimp, and trim the edges as needed. Return to the refrigerator and chill for at least another 30 minutes, but up to 3 hours.