Chocolate Cupcakes with Espresso Buttercream

Preheat the oven to 350 degrees F.
Combine all purpose flour, baking powder, kosher salt, and baking soda in a medium mixing bowl. Whisk together until combined, set aside.
In a separate small bowl, combine cocoa powder, hot milk, and espresso powder. Whisk together and set aside.
Combine sugar, vegetable oil, melted butter, eggs and egg yolk, vanilla extract, vinegar, and buttermilk in a large mixing bowl. Whisk together until combined. Add half of the dry ingredients to the wet ingredients and then pour in the black cocoa mixture. Add the remainder of the flour mixture and mix with a rubber spatula until just combined.
Line a muffin tin with cupcake liners and fill each liner about ⅔ with the cupcake batter. Back for 20-25 minutes. Allow the cupcakes to cool in the pan for 10 minutes, then remove to cool completely on a wire rack.
To make the buttercream, add room temperature butter to a stand mixer with the paddle attachment. Beat on medium until smooth, about 2 minutes. Add powdered sugar, one cup at a time.
Slowly begin to add milk, then add pinch of salt, vanilla extract, and espresso powder.
Add buttercream to a piping bag fitted with your favorite piping tip OR use a spoon to add the buttercream on top of the cooled cupcakes.