Lemonade Party Cake

1 box lemon cake mix
water, oil and eggs as called for on cake mix package
1 can (6oz.) frozen lemonade concentrate, thawed
3/4 c. powdered sugar
Cool Whip

Preheat oven to 350. Prepare and bake cake as directed on package for 13x9 pan. Cool 15 min. Stir together lemonade concentrate and powdered sugar. Poke top of cake every 1/2 inch. Drizzle lemonade mixture evenly over top of cake. Cover and refrigerate at least 2 hours or until chilled. Spread Cool Whip over top of cake. Sprinkle with yellow sugar, if desired. Cover loosely and refrigerate.