Heat oven to 375 degrees F. Line 6 jumbo muffin cups with paper liners
Mix the flour, cocoa powder, baking soda, baking powder and salt together in a bowl. When thoroughly mixed, add the brown sugar and stir well, breaking up any lumps in the sugar.
In a second bowl, mash the bananas, Stir in the eggs, milk and vanilla. Pour the egg mixture into the dry mixture and stir a few strokes, then pour the melted butter in over the egg mixture. Stir until ingredients are well mixed, being careful not to over-beat. Stir in the chocolate chips.
Divide batter equally among muffin cups and bake for 23-26 minutes or until a toothpick inserted in center comes out clean.