Preheat the oven to 400F (200C or 190C fan forced) degrees. Line a 12 cavity muffin tin with muffin papers.
In a large bowl whisk together the flour, cocoa, sugar, baking soda, baking powder and salt. Set aside.
In a second large bowl, whisk together the melted butter, oil, eggs, vanilla extract and buttermilk.
Make a well in the middle of the dry ingredients. Then pour the wet ingredients into the middle of the well. Gently fold the ingredients together using a rubber spatula or wooden spoon.
When the batter is almost combined, gently fold in the chocolate chips.
Spoon the batter into the prepared muffin tin, filling each muffin cavity to the top. You'll end up with about 12-14 muffins in total.
Bake for 5 minutes at 400F. Then turn the oven down to 375F (190C) and continue baking for 15-20 minutes, or until an inserted toothpick comes out clean or with just melted chocolate chip on it. Do not remove the muffins from the oven when you turn the temperature down.
Cool in the pan 5-10 minutes, then continue cooling on a wire rack.