1Heat 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.
2In large bowl, beat reserved 2 tablespoons cake mix, the sugar and eggs with electric mixer on medium speed until creamy. Stir in rhubarb. Pour over partially baked crust. Sprinkle remaining crumbly mixture over top.
3Bake 45 to 50 minutes longer or until golden brown and center is set. Cool slightly before serving. Serve warm or cold with whipped cream. Store covered in refrigerator.