Preheat the oven to 375F and line an 8x8-inch* square baking dish with parchment paper for easy lifting or grease it.
Make the batter:
In a large bowl, place the oats, cocoa powder, baking powder, salt, the ground flax.
On top of the dry ingredients, place the melted coconut or butter, egg (if using instead of flax), vanilla extract, and honey.
Add the mashed banana, peanut butter, and the milk, and start mixing with a fork to combine the ingredients. Continue mixing long enough to dissolve the cocoa powder into the liquid mixture. Add most of the chocolate chips into the batter and mix to combine.
Bake:
Pour the oatmeal mixture into the prepared baking dish, and with a fork, distribute the oats throughout the pan. Sprinkle a few chocolate chips on top.
Bake for 25-30 minutes, until the edges of the oat brownies, are solid and the oatmeal has set in the middle. You can bake this longer for less gooey brownies.
Serve & store:
Remove the pan from the oven and let it cool for a few minutes on the counter. Lift the parchment paper onto a cutting board or counter and once they are cool enough to touch, slice them into squares. Drizzle with a little more peanut butter on top if desired.
Store in the refrigerator in an airtight container for 4-6 days. Reheat in the microwave for 30 seconds and enjoy.
Notes
Baking dish: if using a 9x9-inch square baking dish, check for doneness 5-10 minutes sooner.