Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. In large bowl, place cookie mix. Cut in butter, using pastry blender (or pulling 2 table knives through mix in opposite directions), until mixture looks like coarse crumbs. Reserve 1 1/2 cups of the crumb mixture for topping. Press remaining mixture in bottom of pan. Bake 10 minutes. Cool.