Make the Cupcakes
Preheat Oven & Prep Pan – Preheat oven to 350°F (175°C). Line a 12-cup muffin pan with cupcake liners.
Mix Dry Ingredients – In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Combine Wet Ingredients – In a medium bowl, whisk eggs, oil, buttermilk, and vanilla until smooth.
Mix Wet & Dry – Pour wet ingredients into the dry ingredients and stir until just combined.
Add Coffee & Melted Chocolate – Stir in the hot coffee and melted chocolate until smooth. Batter will be thin.
Fill & Bake – Divide batter evenly among cupcake liners (about ⅔ full). Bake 18–22 minutes, or until a toothpick inserted comes out clean.
Cool Completely – Let cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.
Make the Buttercream
Beat Butter – In a large mixing bowl, beat butter with a mixer on medium speed until creamy, about 2 minutes.
Add Dry Ingredients – Add powdered sugar, cocoa powder, vanilla, and salt. Beat on low until combined.
Add Cream – Pour in heavy cream and beat on high for 3–4 minutes until light and fluffy. Adjust consistency with more cream if needed.
Frost Cupcakes – Transfer buttercream to a piping bag fitted with a large star tip and frost cooled cupcakes.