1. Preheat the oven to 350°F and line a 12-cup muffin pan with cupcake liners.

2. In a large bowl, whisk together the flour, cocoa powder, granulated sugar, brown sugar, baking powder, baking soda, and salt.

3. In a separate bowl, whisk the eggs, milk, vegetable oil, sour cream, and vanilla extract until smooth.

4. Pour the wet ingredients into the dry ingredients and stir gently until mostly combined.

5. Add the hot coffee and mix until the batter is smooth, then fold in the chocolate chips.

6. Divide the batter evenly among the cupcake liners, filling each about two-thirds full.

7. Bake for 18 to 22 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

8. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack and cool completely.

9. To make the frosting, beat the softened butter until creamy.

10. Add the powdered sugar, cocoa powder, melted chocolate, heavy cream, and vanilla extract, then beat until fluffy and smooth.

11. Pipe or spread the frosting over the cooled cupcakes.

12. Finish with extra chocolate chips, chocolate curls, or sprinkles if desired.