Preheat the oven to 350°F. Grease two 9-inch round cake pans, line the bottoms with parchment, and lightly dust the sides with cocoa powder.

Whisk the flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl. In a second bowl, whisk the eggs, buttermilk, oil, and vanilla until combined.

Pour the wet ingredients into the dry ingredients and mix just until no dry streaks remain. Slowly whisk in the boiling water; the batter will be thin, which helps create a moist cake.

Divide the batter between the prepared pans and bake for 27-31 minutes, or until a toothpick inserted in the centers comes out with a few moist crumbs. Cool in the pans for 10 minutes, then transfer the cakes to a rack to cool completely.

Beat the softened butter until pale and fluffy. Add the powdered sugar gradually, then beat in the vanilla and enough heavy cream to make a smooth, spreadable frosting.

Fold the finely crushed Oreos into the buttercream. For the smoothest frosting, crush the cookies into a fine crumb so larger pieces do not drag across the cake.

Place one cooled cake layer on a serving plate and spread it with about 1 cup of Oreo buttercream. Add the second layer, cover the top and sides with a thin crumb coat, and chill for 20 minutes.

Spread the remaining buttercream over the chilled cake, creating smooth sides or decorative swirls. Press extra Oreo crumbs around the base or over the top, then add whole or halved cookies.

Chill the finished cake for at least 30 minutes before slicing. Let it sit at room temperature for 20-30 minutes before serving so the frosting becomes soft and creamy.