Preheat your oven to 325 degrees F. Spray three 8-inch or four 6-inch round cake pans with cooking spray and then line with parchment. Spray again. Set aside.
Place the Oreo cookies in a food processor or blender and pulse until the Oreos are finely ground.
In a medium-size bowl, combine the Oreo crumbs, sugar and melted butter. Stir until all the Oreo crumbs are dampened by the melted butter. You should be able to scoop by hand and have it all stick together.
Divide the mixture evenly between the pans and press down firmly with your hand or the back of a flat measuring cup
Bake for 10 minutes. Let cool while you make the cake layers.
For the pumpkin cake
Keep your oven at 325. In a medium sized bowl, whisk the flour, baking powder, ground cardamon, ground cinnamon, ground nutmeg and salt until combined. Set aside.
In an electric stand bowl mixer fitted with the paddle attachment, cream the sugars and eggs on medium speed until they are light and fluffy. About 5 minutes.
Add the oil and vanilla and beat on medium speed until fully incorporated, about 30 seconds. Add the pumpkin puree and mix on low speed until combined, about another 30 seconds.
With the mixer on the lowest speed, gradually add the dry ingredients to the pumpkin/egg mixture.
Evenly distribute the pumpkin batter to the baked Oreo crust. With the back of a spoon, even out the batter to ensure it’s flat for baking (20 ounces in 8 inch cake pans, about 14 ounces in 6 inch cake pans).
Bake until a toothpick inserted into the center comes out with little to no crumbs on it, about 30-35 minutes. Mine took about 32 minutes.
After they are done baking, let the cakes cool for 20 minutes in the cake pans and then transfer them to a wire rack to finish cooling.
Once they are room temperature, wrap them in plastic wrap and freeze them or put them in the fridge. (They only last 5 days wrapped up in the fridge and up to 6 months in the freezer).
For the cream cheese filling
In the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese for a couple minutes until smooth. Scrape down the sides and beat for 2 more minutes. With the mixer on low, add the granulated sugar. Beat on high for a couple minutes until smooth. Place in a medium sized bowl.
In the same bowl of a stand mixer fitted with the whisk attachment, whisk the heavy whipping cream, and vanilla on high for about 5-7 minutes, until the cream forms stiff peaks.
Fold the whipped cream into the cream cheese mixture until no streaks of whipping cream remain. Making sure not to over mix.
This will hold for a couple of hours at room temperature. If you won’t be using it right away, store in an airtight container in the fridge for up to 2 days.
For the Oreo Pumpkin Buttercream
In the bowl of a stand mixer fitted with a paddle attachment, cream the butter for about 2 minutes. This will soften the butter.
Stop the mixer, add the powdered sugar, pumpkin puree, heavy whipping cream, salt, and the pumpkin pie spice. Turn the mixer on the lowest speed, and mix just until incorporated. Turn the mixer to medium-high speed and beat for an additional 4 minutes. The frosting will become lighter in color and texture.
With a rubber spatula, fold in the pulverized Oreo cookies and beat out the air pockets with a wooden spoon.
Assembly
If needed, level each pumpkin cake layer and then place the first cake layer, Oreo crust side down, on a cake board.
Spread half of the cream cheese frosting onto the cake layer and push out evenly to the sides of the cake.
Add a few crushed Oreos to the top of the frosting and then place another cake layer with the Oreo crust side down. Repeat step 2.
Place the final cake layer, Oreo crust side down, on top and cover with a crumb coat layer of the pumpkin Oreo buttercream. Freeze the cake for 10 to 15 minutes to set the crumb coat.
Finish frosting the cake with the Oreo pumpkin buttercream frosting and serve at room temperature.