FOR THE CAKE

• Preheat the oven to 350 degrees and spray 3- 6″ pans with baking spray.
• Combine the cake mix, buttermilk, eggs, and vegetable oil. Fold in the Oreos. Divide the batter equally among the 3 pans and bake for 27-29 minutes or until lightly golden brown. If the cakes are tested with a toothpick, it should be clean when removed. 

FOR THE MOUSSE

• Using a spatula, fold the whipped topping into the pudding until there are no longer streaks. Keep this in the fridge until assembling the cake. This should be a creamy soft texture, so feel free to add more pudding or whipped topping to make it to your liking. 

FOR THE COOKIE CRUNCH

• Add the Oreo pieces (for the crunch) to a bowl. Melt the milk chocolate according to the package instructions. Stir the chocolate and Oreos until coated.
• Pour the mixture onto a baking sheet lined with parchment paper. Let it set at room temp or move it to the fridge for 10-15 minutes. Once it’s set, chop it into bite size pieces. You want these to be small just to give it a little extra texture. Similar to the crunch of a Dairy Queen ice cream cake (if you’ve had one!)

FOR THE FROSTING

• Beat the butter until smooth. Add in half of the powdered sugar and mix. Add in the rest of the sugar, cream, vanilla, and Oreo crumbs. Beat until smooth and fully combined. If it’s too thick, add more heavy cream 1 tablespoon at a time. It should be easily spreadable, but not runny. 

ASSEMBLY

• Place the first cake layer on a cake board or plate. Spread a thin layer of frosting onto the cake. Using a piping bag with no tip, just cut open, pipe a border around the edge. You want it taller and thick because it’ll hold the filling inside and not leak out through the buttercream. 
• To the center of the buttercream ring, add in the mousse. Fill it about 3/4 tall. Top it with a generous layer of chocolate crunch. Repeat the layers.
• For the final cake layer, flip it over so that the smoother side is on top.
• Apply a layer of frosting to the entire cake. If it’s thick enough you won’t have to put a second layer of frosting on. Spread it with an offset spatula or a cake scraper. Move it to the fridge for 15-20 minutes to set. 
• If you have extra frosting or would like a thicker layer, add it now before you top it with the decorations. 
• Add the remaining frosting to a piping bag with a 1M tip. Place the Oreos at the top of the cake, spacing all 8 evenly apart around the border. Think of it as sort of a clock design around the border. It’ll guide you as to where you put the frosting. Pipe a swirl of frosting in between them all using your piping bag with a 1M tip.
• Slice and serve. 

ALTERNATIVELY

If you’d like to simplify this, bake the cake in 2- 8″ cake pans instead. Once cooled, spread a layer of frosting on the first cake layer. Create a frosting border on top. Basically add a thick border of frosting on the outside border of the cake so that the filling doesn’t come out. If you don’t have piping bags, you can use a plastic bag! It doesn’t have to be pretty it’s just securing the filling. 

Add the mousse and crunch and top it with the other layer. Flip it so that the smooth side is on top. Add a coat of frosting on the outside and you’re done! 

You’ll probably need less frosting for the second option. 2 sticks of butter, 4 cups of powdered sugar, 1 tsp vanilla, 4-5 tbsp heavy cream, 1/2 cup Oreo crumbs.