Preheat oven to 350°F (175°C) and line a muffin tin with cupcake liners.
In a bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, cream butter and sugar until light and fluffy.
Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
Alternately add the dry ingredients and milk to the butter mixture, beginning and ending with the dry ingredients.
Fold in crushed Thin Mint cookies.
Fill cupcake liners 2/3 full and bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Allow cupcakes to cool completely before frosting.
For the Frosting: Beat butter until smooth, then gradually add powdered sugar.
Mix in peppermint extract and heavy cream until light and fluffy. Add green food coloring if desired.
Frost cupcakes and garnish with chocolate shavings and crushed Thin Mint cookies.