Preheat the oven to 375 degrees F. Grease muffin tins or add muffin liners.
In a medium bowl, stir together the flour, ground flax seed, baking soda, and salt. Set aside.
In another medium bowl, beat together the bananas and oil. Add the brown sugar and egg, continuing to beat until completely combined. Add the wet ingredients to the dry ingredients and combine until just blended. Add the chocolate chips if desired.
Spoon the batter into the prepared muffin tins, filling each about 3/4 full. Bake until a toothpick inserted comes clean, about 15 to 20 minutes. Cool in the pan for 5 minutes before turning out onto a cooling rack.