Heat the oven to 350℉/177℃.
Line a cupcake tin with paper liners or spray it with baking spray. Set aside.
Whisk the cocoa powder and boiling water together in a small bowl. Set aside.
In a large bowl, whisk the flour, sugar, baking powder, and salt together.
Add the milk, oil, egg, egg yolk, and vanilla.
Whisk until smooth.
Whisk in chocolate.
Use a 3 Tablespoon scoop to portion the batter into your prepared pan.
Bake for 18-20 minutes, or until the cupcakes test done.