Heat oven to 350°F. Line a 12-cup muffin pan with paper liners.
Whisk flour, black cocoa, regular cocoa, sugar, baking soda, baking powder, and salt in a large bowl.
Blend buttermilk, oil, eggs, vanilla, vinegar, and black gel color in a separate bowl.
Pour wet mixture into dry ingredients. Stir gently until just combined and a few streaks remain.
Whisk in hot coffee until the batter is smooth and pourable.
Fill liners about two-thirds full with batter.
Bake 17–20 minutes, until a toothpick comes out with a few moist crumbs.
Cool cupcakes in the pan 10 minutes, then cool completely on a rack.
Beat softened cream cheese and butter until creamy and fluffy, about 2 minutes.
Add powdered sugar, vanilla, and salt. Mix on low, then whip until light.
Drizzle in heavy cream as needed for a smooth, spreadable frosting.
Swirl frosting onto cooled cupcakes. Garnish as you like.