Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, mix the egg, buttermilk, vegetable oil, and vanilla extract until well combined.

Gradually add the wet ingredients to the dry ingredients and mix until just combined. Fold in the crushed Oreo cookies.

Divide the batter evenly among the cupcake liners and bake for 15-20 minutes or until a toothpick comes out clean.

Allow cupcakes to cool completely before frosting.

In a mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar and heavy cream, mixing until smooth.

Stir in the vanilla extract and mix until well combined.

Frost the cooled cupcakes and top with crushed Oreo cookies.