Heat oven to 350. Prepare cake batter as directed on box and bake in 2-9 in. round pans. Cool cakes 10 mins. in pan when done baking and cool completely on wire racks.
If you are going to use chocolate topping, microwave butter and chocolate until melted, stir until smooth and cool 5 mins.
Blend cream cheese and sugar. Gentley stir in whipped toppings and oreo cookies. Place one cake on serving plate and top with cream cheese mixture. Top with other cake. Pour chocolate topping over top of cake and let it run down the sides. No need to spread unless you want a neater look cake! Let stand until firm. Keep cake refrigerated.