Method:
Put butter into a small bowl and freeze while you prepare other ingredients. Mix the flours with salt in a medium bowl or food processor. Add cold margarine and cut in using a pastry blender, or by pulsing in the food processor. Add 2 to 3 tablespoons ice water, about 1/2 tablespoon at a time, until dough forms into a ball. Gather up and pat out into a disc. Cover with plastic and refrigerate for 30 minutes, if possible. When ready to use, roll dough out on a lightly floured board into a 10-inch circle. Gently fold into quarters using a little flour as needed to prevent sticking. Place dough into a 9-inch pie plate and carefully unfold, fitting loosely and then pressing into place. Trim and crimp edges for a decorative crust.