Preheat your oven to 220° C (425° F), line a 12 cup muffin pan with 12 paper liners.
In a mixing bowl, add the milk and vinegar, stir together. (You are making buttermilk.)
In another mixing bowl, add the flour, cocoa powder, sugar, baking powder, baking soda, and the salt. Whisk together.
To the milk mixture, add the melted butter, eggs, and oil. Whisk together.
Add the wet mixture to the dry mixture and mix until almost combined.
Add the chocolate chips and mix in. Now the batter is very much like a cake batter.
Spoon the batter into the 12 cup cake liners equally. Sprinkle a few chocolate chips on top of each cup.
Bake for 5 minutes then reduce heat in the oven to 180° C (350° F) and continue baking for another 13 to 16 minutes or until a toothpick inserted in the center of a muffin comes out clean. (Mine were perfect at 13 minutes.)
Remove muffins to a rack to cool completely.
Serve and enjoy.