Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or line the bottoms with parchment paper.
Step 2: In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.
Step 3: Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Mix on medium speed with a hand mixer or stand mixer until smooth.
Step 4: Carefully pour in the boiling water and stir by hand until the batter is thin and consistent. Fold in the 15 coarsely crushed Oreo cookies.
Step 5: Divide the batter evenly between the prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Step 6: Allow cakes to cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.
Step 7: To make the frosting, beat the softened butter until creamy. Gradually add powdered sugar, heavy cream, and vanilla. Once smooth, fold in the 10 finely crushed Oreo cookies.
Step 8: Frost the cooled cake layers, stacking them with a thick layer of frosting in between, and decorate with whole or halved cookies on top.