Preheat your oven to 350°F (175°C).
Line a muffin tin with parchment paper liners or spray it with non-stick cooking spray.
In a large mixing bowl, combine the melted butter and sugar. Whisk until well blended. Add the eggs and vanilla extract, mixing until smooth and creamy.
In another bowl, whisk together the flour, cocoa powder, baking powder, and salt until well combined.
Gradually add the dry mixture to the wet ingredients, stirring just until combined.
Gently fold in the semi-sweet chocolate chips.
Using a spoon or an ice cream scoop, fill each muffin cup about three-quarters full with batter.
Place the muffin tin in the preheated oven and bake for 18-20 minutes.
Remove the muffins from the oven and let them cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.