Preheat oven to 350°F.
Line a 12 well cupcake pan with cupcake wrappers. Spray with baking spray. Set aside.
In a bowl, whisk together the cake flour, black cocoa powder, baking powder, baking soda, espresso powder, and salt.
Next, add the buttermilk, sour cream, and vinegar into a bowl and whisk to combine.
Add the butter and sugar into a large bowl and beat on high speed with an electric mixer until smooth and fluffy. This takes 3 to 5 minutes. Mix in the egg, the egg yolk, and the vanilla.
Slowly pour in the liquid black velvet cupcake mixture and continue to mix on low speed until just combined. Carefully mix in the dry ingredients until just combined.
Using a scoop, fill the prepared cupcake wells 3/4 full of the black velvet cupcake batter.
Bake for 18 to 22 minutes in the preheated oven.
Make the Black Buttercream while the cupcakes are baking. The Black Velvet Buttercream Free Printable Recipe is in the Notes below with flavoring options and info about black cocoa powder.
Remove the black cupcakes from the oven and cool on a baking rack for 30 minutes or until cooled before frosting.