From the blog post, where you can get the recipe:
The cupcake recipe is a nice time-saver as it uses a chocolate boxed mix for the base with a few additions (still comes out super tasty!) and I paired it with my basic buttercream recipe with finely crumbed cool mint oreos and peppermint flavoring added in.
From An American Cupcake in London:
I used a funfetti cupcake mix, which made 6 cupcakes. I put one spoon of batter into the liner and placed the Oreo in and then placed another spoon of batter on top. I made half of this frosting recipe with some vanilla paste and no food coloring.