In a food processor, combine the flour, salt and baking powder and pulse to mix. Add the butter and shortening and process just until the mixture resembles coarse meal. With the machine on, slowly add 4 tablespoons of the ice water and pulse just until the dough is evenly moistened and comes together; add more water if necessary. Transfer the dough to a work surface, then gather it into a ball. Pat the dough into a disk, wrap it in plastic and refrigerate for at least 1 hour.