Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, sift together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Add eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until well combined.
Carefully stir in the boiling water. The batter will be thin; this is normal.
Fold in the finely crushed Oreo cookies until evenly distributed.
Divide the batter evenly between the prepared cake pans.
Bake for 30 to 35 minutes or until a toothpick inserted into the center comes out clean.
Remove from oven and allow cakes to cool in the pans for 10 minutes. Then remove from pans and cool completely on wire racks.
To make the Oreo frosting, beat the softened butter on medium speed until creamy, about 2 minutes.
Gradually add powdered sugar, beating on low speed until combined.
Add heavy cream and vanilla extract, then beat on high speed for 3 minutes until light and fluffy.
Fold in 1 cup of finely crushed Oreo cookies into the frosting.
Place one cake layer on a serving plate. Spread a thick layer of Oreo frosting on top.
Place the second cake layer over the frosting and cover the entire cake with the remaining frosting.
Decorate the top with whole Oreo cookies as desired.
Refrigerate the cake for at least 30 minutes before serving to set the frosting.