Make the Chocolate Ganache Filling: Heat ½ cup of heavy cream in a heatproof bowl in the microwave for 45 seconds, until it just begins to bubble and steam. Gently pour 2 cups of milk chocolate chips into the heavy cream. Let the mixture sit for 1 minute. Stir slowly until the cream and chocolate combine and the mixture is smooth. Mix in ¼ cup of room temperature unsalted butter, 2 Tbsp corn syrup (optional), and ½ tsp fine salt until combined. Cover with plastic wrap and set aside to cool to room temperature.

Preheat oven to 350°F/175°C. Line three 8-inch cake pans with parchment paper rounds and spray the sides with nonstick baking spray.

Sift or whisk together 2 cups flour, 2 cups granulated sugar, ¾ cup black or dark cocoa powder, 2 tsp baking powder, 1 ½ tsp baking soda, 1 tsp fine salt, and 1 tsp instant espresso or coffee (optional) in a large bowl.

In a separate bowl, add 1 cup warm water, 1 cup buttermilk, ½ cup oil, 2 eggs, and 1 tsp vanilla and whisk together until fully incorporated. Whisk the dry ingredients into the wet ingredients until they're just combined.

Pour the batter evenly between the 3 cake pans. Bake for 24-26 minutes or until a toothpick comes out with a few moist crumbs. Let the pans cool for 10 minutes, then run a small offset spatula around the perimeter of the pan to separate the cake from the pan. Place the cake pans into the freezer for 30 minutes to accelerate the cooling process.

Once the layers are fully cooled, carefully flip the pans and remove the layers. Level them with a serrated knife if needed.

Beat 2 cups of unsalted butter on medium speed for 30 seconds until smooth. Mix in 1 Tbsp vanilla bean paste or vanilla extract and ½ tsp salt on a low speed. Gradually add in 5 cups of powdered sugar while mixing slowly. Halfway through, add ¼ cup of heavy cream.

Scoop half of the frosting into a separate bowl and fold in 1 ½ cups of roughly chopped Oreos. Mix ⅓ cup of unsweetened cocoa powder and the remaining 2 Tbsp of heavy cream into the other half of the frosting. Then mix 1 cup of melted and cooled milk chocolate into the frosting.

Place the first cake layer on a greaseproof cake board or large flat plate. Spread half of the room-temperature chocolate ganache on the first cake layer, then spread half of the Oreo buttercream on top. Add the second cake layer and repeat this process, then place the final cake layer upside down. Chill the cake for about 15 minutes in the freezer or 30 minutes in the fridge.

Cover the cake in a thick layer of chocolate buttercream. Use a large offset spatula to create a textured look around the cake. Top the cake with a ring of chopped Oreos.