Preheat oven to 350°F (175°C) and line a muffin tin with cupcake liners.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients and mix until well combined.
Carefully add the boiling water to the batter and mix until smooth. The batter will be thin.
Pour the batter into the prepared muffin tin, filling each liner about ⅔ full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cupcakes to cool in the tin for 10 minutes, then transfer to a cooling rack to cool completely.
To make the frosting, beat the softened butter in a large bowl until creamy.
Gradually add the powdered sugar, milk, and peppermint extract, beating until smooth and fluffy.
Fold in the mini chocolate chips.
Frost the cooled cupcakes with the mint chip frosting.