If you are not using buttermilk, in a bowl mix milk and vinegar and set aside.
In a mixing bowl place flour, cocoa powder, baking powder, salt, brown sugar and chopped chocolate. Mix well.
Add to this mixture, vegetable oil, vanilla extract, eggs and buttermilk (or milk and vinegar mixture). Mix well.
Pour batter in prepared muffin cups and take them to the oven and bake for around 15 minutes or until a toothpick inserted in the middle of one of the muffins comes out clean (see note 1).
Notes
Note 1: If in doubt if ready, test another muffin. If you hit a chocolate piece, you might get the impression they will need extra time in the oven.Adapted from Everyday Easy Cakes & Cupcakes Cookbook