Preheat oven to 350F/170C. Spray a square 8x8" baking pan with cooking spray and line with parchment paper for easier removal.
In a large bowl, mash the bananas. (Be sure to measure if it's 1 cup.) Add the egg and whisk slightly with a fork to incorporate.
Next, using a spatula, stir in the oil, milk, vanilla and brown sugar until completely combined.
Then add the flour, oats, sifted cocoa powder, ground flax seeds, baking soda and salt and fold gently just until everything is incorporated.
Last, fold in the chocolate chips and pour batter into the prepared batter, smoothing it out evenly.
Bake in the preheated oven for about 18-20 minutes until it's set and springs back to the touch a bit. Be sure not to over-bake it for moist and gooey bars. Let cool slightly and slice into squares. Enjoy.
This can be kept covered at room temperature for 2 days or stored in the fridge for up to a week.
*I wouldn't recommend olive oil as it will give a slightly stronger flavor than vegetable, coconut, or another milder oil like grapeseed. But do let me know if you try it with success.