Preheat oven to 400°. Grease a 12-cup muffin tray.
In a large bowl, stir together flour, brown sugar, cocoa powder, baking powder, baking soda, and salt.
In a medium bowl, whisk together buttermilk or sour milk, sour cream, eggs, and butter. Make a well in the flour mixture and add liquid to centre. Stir to somewhat combine ingredients, then add chocolate chips. Stir just until moistened (batter will be lumpy).
Spoon batter into prepared pans, filling 3/4 full. Bake 15-20 minutes until tops are firm. Cool on wire racks for a few minutes before removing from pan and allowing to fully cool, or serve warm.