Heat oven to 350℉/177℃.
Line muffin tin with paper liners, or spray with baking spray. Set aside.
In a small bowl, whisk together cocoa and boiling water. Set aside.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together egg, egg yolk, oil, milk, and vanilla.
Pour wet ingredients over dry ingredients.
Whisk only to blend.
Stir in chocolate.
Fill muffin cups ⅔ full.
Bake for 18-20 minutes, or until done.