Preheat the oven to 350°F (175°C).
In a large mixing bowl, whisk together the dry ingredients: flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In another bowl, combine the melted butter, eggs, vanilla extract, and buttermilk, whisking until fully combined.
Pour the wet mixture into the dry ingredients, and gently mix until just combined.
Fold in the chocolate chips, if using.
Line a muffin tin with liners and fill each cup about 2/3 full with batter.
Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
