PREHEAT oven to 350°F. Coarsely chop 15 of the cookies. Beat cake mix, water, oil and egg whites in large bowl with electric mixer on low speed just until moistened. Beat 2 minutes on high speed. Stir in chopped cookies. Pour into greased and floured 13x9-inch baking pan.
CUT 8 of the remaining cookies in half; arrange in rows in trianglular shape over batter to resemble a tree, pressing cut side of each cookie gently into batter. Place 1 of the remaining whole cookies at the top of the tree for the star and remaining whole cookie at the bottom of the tree for the trunk.
BAKE 30 to 40 minutes or until toothpick inserted in center comes out clean. Cool completely in pan on wire rack. Remove cake from pan; place on large plate or tray. Decorate with decorating icing and candies as desired. Sprinkle with powered sugar. Cut into 16 squares to serve.
