Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom of 8- or 9-inch square pan with shortening or cooking spray. In medium bowl, stir all ingredients except frosting until well blended.
Spread in pan.
Bake 8-inch pan 35 to 38 minutes, 9-inch pan 28 to 31 minutes, or until toothpick inserted 2 inches from side of pan comes out almost clean. Cool completely on cooling rack, about 1 1/2 hours.