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.
Preheat oven to 350 degrees F. Grease a mini muffin pan with cooking spray. Fill the muffin cups with 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.