Preheat and prep: Heat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Mix dry ingredients: In a large bowl, whisk flour, sugar, cocoa powder, baking powder, baking soda, and salt until uniform.
Combine wet ingredients: In a separate bowl, whisk buttermilk, oil, egg, and vanilla until smooth.
Bring batter together: Pour wet ingredients into dry.
Stir gently until just combined. Add hot coffee or water and whisk briefly until the batter is smooth and glossy. It will be thin; that’s okay.
Fill liners: Divide batter evenly among liners, filling each about two-thirds full.
Bake: Bake 16–19 minutes, until a toothpick comes out with a few moist crumbs.
Do not overbake.
Cool: Let cupcakes rest in the pan 5 minutes, then transfer to a rack to cool completely before frosting.
Make mint frosting: Beat softened butter until creamy. Add powdered sugar, 1 tablespoon cream, peppermint extract, and a pinch of salt. Beat until fluffy, 1–2 minutes.
Adjust consistency with more cream as needed. Add a drop or two of green coloring if you like.
Frost and finish: Swirl frosting onto cooled cupcakes. Top with mini chocolate chips or shavings for texture.