Heat oven to 350°F (325°F for dark or nonstick pan). Reserve 2 tablespoons of the cake mix. In large bowl, cut butter into remaining cake mix, using pastry blender (or pulling 2 table knives through ingredients in opposite directions), until crumbly. In bottom of ungreased 13x9-inch pan, pat 2 cups of the mixture. Reserve remaining crumbly mixture for topping. Bake 13 minutes.