Heat oven to 375°F. In medium bowl, mix 1/2 cup all-purpose flour, the whole wheat flour and 1/4 teaspoon salt. Cut in butter, using pastry blender (or pulling 2 table knives through ingredients in opposite directions), until mixture looks like small crumbs. Sprinkle with water, 1 tablespoon at a time, tossing with fork just until flour is moistened. Shape into ball; flatten slightly.