Preheat the oven to 350 degrees. Coat a 13-by-9-by-2-inch pan with nonstick cooking spray.

Prepare cake. In a large mixing bowl, combine the cake mix, buttermilk, egg whites, egg and oil; beat until well-mixed.

In a small mixing bowl, combine the brown sugar, coconut and pecans; set aside.

Spread half the batter in the bottom of the prepared pan. Sprinkle with the brown-sugar mixture. Carefully top with the remaining batter, spreading it out. Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean. Let cool.

When cake cools, prepare the frosting. In a medium mixing bowl, beat the cream cheese and margarine together. Blend in the powdered sugar, mixing well. Mix in vanilla.

Spread cooled cake with frosting. Refrigerate.