Preheat your oven to 325 degrees. Spray three 8-inch cake pans with nonstick cooking spray, line the bottoms with parchment paper, and spray again. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment Combine the flour, sugar, cocoa powder, and salt Mix on low until evenly combined.
Add the melted butter and mix on low until the mixture starts to come together and forms small ball clusters.
Spread 7 ounces of the chocolate crumbs on the parchment in each of the three 8-inch cake pans. Using the back of a spoon or your finger tips, press the crumbs together to form a crust. Bake for 6 to 8 minutes.
For the cake
Keep your oven at 325 degrees.
In a medium sized bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugar on high until fluffy and smooth, about 4 minutes.
Beating on medium speed, start adding the whole egg and the egg whites, one at a time to the bowl, scraping the bottom and the sides of the bowl between each addition. After all the eggs have been added, beat on medium speed for 1 minute. Add the sour cream and vanilla, beat on medium speed for 30 seconds.
Turn the mixer on low and add half of the flour mixture and half of the buttermilk to the butter mixture and mix until fully incorporated. Repeat by adding the rest of the flour mixture and the rest of the buttermilk, while mixing on low.
With a rubber spatula, fold in the mini chocolate chips.
Using a kitchen scale, evenly distribute the cake batter among the three 8-inch or four 6-inch pans (about 18 ounces in the 8 inch, and 12 ounces in the 12 inch) and bake for 25-30 minutes, or until a toothpick comes out with just a few moist crumbs on it or until it starts to get a little golden brown on top.
Remove cakes from the oven and let them cool in their pans for 15 minutes before taking them out of the pans and placing them on cooling racks.When the cakes are completely cooled, wrap each cake layer individually with plastic wrap and stick it in the refrigerator or freezer until you’re ready to frost.If your cakes have a small dome, level them when they are cool, but before you place them in the fridge.
For the Cookie Dough
With an electric hand mixer, cream the butter and sugars until light and fluffy, about 3 minutes.
Add the vanilla extract and mix on low until combined.
Add in the flour and salt, mix until the dry ingredients are incorporated.
With a rubber spatula, mix in the mini chocolate chips.
Set aside ⅔ of the dough for the cake filling and use the remaining ⅓ to roll into small balls for decorating (I rolled mine into 1 ounce balls and dipped the balls in melted chocolate).
With the other ⅔s of the cookie dough, divide it into 2 even parts. Place a generous layer of plastic wrap inside an 8 inch cake pan. Take one of the pieces of cookie dough and evenly spread to it the edges of the cake pan, making a cookie dough layer to use later. Repeat 2 more times and set aside.
For the brown sugar buttercream
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high until creamy, about 3 minutes. With a rubber spatula, scrape down the sides of the bowl.
Once the butter is creamy, add the brown sugar and beat on medium-high until combined, about 1 minute.
Add the powdered sugar, heavy cream, vanilla extract, and salt. Turn the mixer to low and mix just until combined, about 30 seconds. Turn the mixer up to medium-high and beat until completely combined and creamy.
With a rubber spatula, beat out the air pockets by hand until smooth and creamy.
Assembly
Spread a small dollop of buttercream on the middle of the cake board. Place the first cake layer on the board, with the chocolate cookie crust on the bottom.
Place prepped cookie dough right on top of the cake layer.
Place the second cake layer, chocolate cookie crust on the bottom, and repeat step 2.
Place the final cake layer, chocolate cookie crust on the bottom, on top of the second layer of cookie dough.
Spread a thin layer of buttercream around the entire cake to lock in any crumbs. This is called the crumb coat. Freeze the cake for 10 minutes to ensure the crumbs get locked into this coat of buttercream.
Frost the cake with the remaining brown sugar buttercream and top with the chocolate dipped cookie dough balls.