Heat oven to 350ºF. Line bottom and sides of 9-inch square pan with foil, leaving 1 inch of foil overhanging at 2 opposite sides of pan. In medium bowl, place flour. Cut in butter using pastry blender (or pulling 2 table knives through ingredients in opposite directions), until evenly mixed. Stir in nuts. In bottom of pan, press mixture evenly. Bake about 15 minutes or until light golden brown. Cool completely.