Heat the oven to 450ºF. If you're measuring the flour by volume, do so by spooning it into a measuring cup and leveling it off. Put the flour into a large metal mixing bowl, add the baking powder and salt, and mix well with a whisk. Cut in the butter with two knives or a pastry blender until it's uniformly mixed in and there are no large lumps. Add 1-1/4 cups of the buttermilk, stirring with a rubber spatula until the mixture is just blended and leaves the sides of the bowl. Don't overmix. Add more buttermilk or flour only if necessary.
Dump the contents out onto a lightly floured surface. Place the fingers of both hands down inside the flour bag to coat them. Using only your fingers, lightly pat the dough together. With a floured rolling pin, lightly even out the dough to 3/4 inch thick. Using a floured metal 2-1/2-inch biscuit cutter -- not an overturned glass, which will seal the edges so they can't rise -- quickly punch out the biscuits. Don't twist the cutter in the dough (which also seals the edges). Avoid touching the dough with your hands. Gather any scraps, roll, and cut out more biscuits. These won't rise as high, but they'll still be quite good.
Flour your fingertips before patting out the dough. Finish with a rolling pin, using a light touch.
Punch (don't twist) your biscuits. Use a metal cutter, not an overturned glass.
Set the biscuits, close but not touching, on an ungreased baking sheet and bake until they're lightly browned on top, 15 to 18 min. Serve at once with the ham.