Preheat the oven to 425°F and line a muffin tin with 12 cupcake liners. If you don’t have cupcake liners, you can also grease the muffin tins very well with butter!
In a large mixing bowl, combine the melted butter and granulated sugar. Whisk to combine. Add the eggs, one at a time and whisk until frothy.
Add the vanilla extract, greek yogurt and buttermilk, and stir together.
Add the flour, cocoa powder, baking powder, salt. Mix until just combined. Do not over mix. Last, add the chocolate chips and mix once or twice to distribute.
Use a large ice cream scoop or a 1/3 cup measure to divide the batter evenly between the 12 muffin tins. Bake at 425° for 5 minutes. After 5 minutes, reduce the oven temperature to 375°F and bake for 15 more minutes or until a knife inserted into the middle of the muffins come out mostly clean.