Place the Oreo cookies in a zip-top bag and crush with a rolling pin. Set aside.

Prepare the cake mix according to the package instructions. Pour in half the crushed cookies and set the remaining cookies aside.

Bake the cake in a 9x13 baking pan according to package instructions.

When the cake is finished cooking, remove the pan from the oven and allow to cool completely.

To prepare the icing beat together the vanilla extract and butter until smooth. Add the powdered sugar and one tablespoon of milk, then beat to combine. Add more milk, if needed, if the icing is too thick.

Spread the icing over the top of the cooled cake and top with remaining crushed cookies.