Heat oven to 350°F. Line 13x9-inch pan with foil, leaving 1 inch of foil overhanging at 2 opposite sides of pan. In medium bowl, stir together cake mix and 1/2 cup pecans. With pastry blender or fork, cut in butter until mixture is crumbly. Lightly press in bottom of foil-lined pan. Bake 15 to 20 minutes or until set and just beginning to brown. Cool completely, about 30 minutes.