Mix cake per directions on box using all ingredients listed under Cake.. except the Oreos.
Divide the batter evenly between the pans.Bake for approximately 25- 28 minutes or done.
Cool on rack till completely cool so it won't melt your Cool Whip.
Melt and cool the stick of butter.
Add Vanilla to cooled butter and stir in Cool Whip. The butter will thicken the mixture to Frosting consistency. Set it aside.
In a separate bowl break up enough of the Oreo cookies to spread between your cake layers. Make them pretty big pieces. I used 10 cookies. Use the whole cookie. Filling and all. Set it aside.
In another bowl. Use the whole cookie, filling and all. Blend or process 6-8 cookies to crumbs so you can decorate the sides of your frosted cake. It won't take a lot. Set it aside.
TO ASSEMBLE THE CAKE:
Place 1 cake layer on the serving plate and spread with a large dollop of the cool whip Frosting. Use plenty and sprinkle broken cookie pieces on top of the cool whip.
Add the top layer and frost top and sides with the remaining cool whip frosting. Save enough to fill a small piping bag if you are going to decorate the top with rosettes.
I used a # 32 Wilton star tip on my bag.
I cut cookies in half to decorate the top of cake. To keep them from breaking so badly I separated each cookie and cut them in half with a very sharp knife. Then put the halves back together and stuck each half in a rosette of frosting.
Hold the plate at an angle over the container of crumbs and sprinkle as much as you want on sides of cake.