For muffin batter:
In a large bowl, mix together the eggs, baking powder, cocoa powder, vanilla extract, and salt until well combined. Stir in the sugar and melted butter until well combined.
Slowly add the flour and mix with a firm rubber spatula until combined. Set the muffin batter aside.
In another bowl, beat the cream cheese, sugar and egg yolk until smooth. Set the filling aside.
Preheat oven to 350 degrees F. Grease a mini muffin pan with cooking spray. Fill the muffin cups 1/2 full with the muffin batter.
Top with a teaspoon of the filling.
Then top each cup with an additional teaspoon of the muffin batter.
Bake until a toothpick inserted comes out clean, about 14-16 minutes. Remove from the oven and allow to cool 10 minutes in the muffin pan. Then transfer to a wire rack to cool completely.