Preheat oven to 350 degrees and line two 9" pans with parchment paper and spray with cooking spray. In a large bowl mix everything together for chocolate cake. We will repeat all these steps for the vanilla cake.
Pour batter into prepared pans and follow the cooking time instructions on the back of the dark chocolate cake box. You may need to add about 10 minutes extra to whatever it says. You can always check with a toothpick to see if it's done.
Remove pans and let rest 5 minutes on a cooling rack before removing the cakes. let cakes cool completely before frosting.
Repeat all steps for vanilla cake, combining all the ingredients (cake mix, pudding, sour cream, eggs, oil, and milk. Mix together and bake according to the instructions on the box.
In a food processor or a zip lock bag, blend or crush Oreos until they turn into a powder consistency.
Mix butter with powdered sugar a 1/2 cup at a time until all blended. Add vanilla and heavy cream and mix well. Add crushed Oreos and blend.
if the mixture is too thick, add an additional teaspoon of heavy cream one at a time until desired thickness and consistency is achieved.
Spread between each layer of cake and all over the sides and top. I simply used a zip lock bag (cut a small corner off and piped on the small twirls of frosting on the top. Decorate with whole Oreo cookies til you achieve the final look you like.