Heat oven to 350°(180°C) (325°F/160°C for dark or nonstick pan). Reserve 2 tablespoons (30 mL) 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 (33x23 cm) pan, pat 2 cups (500 mL) of the mixture. Reserve remaining crumbly mixture for topping. Bake 13 minutes.