Step 1: Preheat your oven to 350°F (175°C) and grease three 8-inch round cake pans, lining the bottoms with parchment paper.
Step 2: In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Step 3: Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Mix on medium speed until well combined.
Step 4: Carefully pour in the boiling water and stir by hand until the batter is smooth. The batter will be thin.
Step 5: Gently fold in the 20 coarsely crushed Oreos until evenly distributed.
Step 6: Divide the batter equally between the prepared pans and bake for 30-35 minutes, or until a toothpick comes out clean.
Step 7: Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
Step 8: Prepare the frosting by beating the butter until pale and fluffy. Gradually add powdered sugar, heavy cream, and vanilla.
Step 9: Fold the finely crushed Oreos into the frosting.
Step 10: Assemble the cake by frosting between each layer and then covering the top and sides. Garnish with whole Oreos if desired.