Preheat oven to 350°F. Line muffin tin with 12 cupcake liners.
Combine the flour, ground flax seed, cocoa powder, baking powder, baking soda, & ¼ cup chocolate chips in a medium sized bowl.
In a separate smaller bowl, whisk oil, mashed banana, sugar, egg, & milk together.
Gradually pour wet ingredients into dry gently stirring. (You do not want to over mix or it will be dry). Stir until all ingredients are just combined.
Pour ~1/4 cup batter into each muffin liner. Top with remaining ¼ cup chocolate chips on top of muffins. Bake at 350°F for ~15 minutes or until a few crumbs come out when doing a toothpick test.
Note: Do not overbake or they will be dry.