While pulsing, slowly pour in 1/2 cup of the buttermilk (1/4 for half batch, 2 tablespoons for a quarter batch) and continue to pulse until the buttermilk is incorporated into the dough. At this point, your dough should look like rough crumbs and just hold its shape if squeezed together. If the dough does not, add a a little more buttermilk, a teaspoon at a time.
Lay a sheet of plastic wrap on the counter and dump half of your dough crumbs onto the sheet. (If you are making only a single crust, do not divide dough.) Gather the edges and press tightly together to form a 1-inch thick disk. Do the same with the rest of the dough. Place in a plastic bag and refrigerate for at least an hour, up to 24.
*If you measured your flour by weight, you will need closer to 1/2 cup of buttermilk for a full double crust, but if you measure your flour by volume, you will almost certainly technically have more than 2 1/2 cups of flour (scooping flour into measuring cups tends to compress it), so you will probably end up needing closer to 2/3 cup. Use just enough buttermilk for your dough to hold together. Using too much can result in a tough, crunchy finished product.