Preheat the oven to 350 degrees f. Combine black cocoa powder and hot water in a large bowl and whisk together and let it sit for about 5 minutes, then add sugar, vegetable oil, and melted butter. Whisk for about 1 minute, until combined, then add eggs, egg yolk, vanilla extract, vinegar, and buttermilk. Continue to whisk until combined, about 1 minute. 2/3 cup Water2/3 cup Black Cocoa1 cup Sugar1/3 cup Vegetable Oil5 tbsps Salted Butter2 large Eggs1 Egg Yolk1 1/2 tsp Vanilla Extract1 tsp Vinegar1/2 cup Buttermilk
Add all purpose flour, baking powder, baking soda, and kosher salt. Whisk together until just combined and all of there's not dry patches of flour. 1 1/3 cup All Purpose flour1 tsp Baking Powder1/4 tsp Baking Soda1/2 tsp Kosher Salt
Line a muffin tin with cupcake liners and fill each liner about 2/3 with the cupcake batter. Back for 18-20 minutes, until a toothpick inserted into the center of a cupcake comes out with just a few moist crumbs. Allow the cupcakes to cool in the pan for 5 minutes, then turn over onto a wire rack to cool completely.
Make the black cocoa frosting - melt chocolate chips and corn syrup in the microwave in 15 second increments until melted. Set aside. 1 cup Semi-Sweet Chocolate Chips4 tbsps Corn Syrup
In the bowl of a stand mixer fitted with the paddle attachment, beat butter for 2 minutes, then add cream cheese and beat together for 1 minute. Then add powdered sugar and black cocoa. Continue to mix together until combined, then add melted chocolate mixture and heavy cream. 1 cup Salted Butter4 oz Cream Cheese1 cup Powdered Sugar1/2 cup Black Cocoa Powder1 tbsp Heavy Cream
Scrape down the sides of the bowl of your stand mixer and continue to mix for another minute. You can add a couple drops of black food coloring for a true black frosting, but this is optional.
Add the black cocoa buttercream to a piping bag and pipe the icing onto the cooled cupcakes.