Preheat oven to 350F and line 18 muffin cups with paper liners.
Whisk together flour, granulated sugar, cocoa powder, baking soda, and salt in a medium bowl.
In a large bowl, whisk hot coffee, oil, buttermilk, egg, vanilla, and vinegar until smooth.
Add dry ingredients to wet ingredients and fold with a spatula just until combined.
Fill muffin cups 2/3 full with batter.
Bake 20-23 minutes until a toothpick comes out clean or with moist crumbs.
Cool cupcakes completely on a wire rack.
Microwave chocolate in 30-second intervals, stirring between each, until melted and smooth, then set aside to cool slightly.
Beat softened butter with an electric mixer on medium-high speed until creamy and pale yellow.
Sift confectioner's sugar and cocoa powder together, then add to butter on low speed.
Pour in cooled melted chocolate and vanilla, then beat until smooth and fluffy.
Pipe or spread frosting onto cooled cupcakes.