Add the mashed banana, egg, vanilla, and brown sugar to the bowl with the butter. Blend the ingredients together with an electric mixer.
In a second bowl combine the flour, oats, cinnamon, baking soda, and salt. Whisk together.
Slowly add some of the dry ingredients to the wet ingredients bowl and use an electric mixer on low to combine (or stir by hand with a wooden spoon).Continue until all of the dry ingredients have been added to the wet ingredients. The dough should be combined but not completely mixed. Usea rubber spatula to scrape sides and stir in ¾ cup of the chocolate chips. (reserve ¼ cup for later)
Cover the bowl of dough with plastic wrap and store in the refrigerator for a minimum of two hours or overnight.
Preheat oven to 350 degrees. Line a cookie sheet with parchment paper or a silicone mat. Measure the cookie dough balls using a two tablespoon scoop and place the dough balls 2 inches apart on prepared cookie sheet. Top with a few chocolate chips on each cookie.
Bake at 350 degrees for about 10 to 12 minutes. The cookies will be slightly undercooked. Remove from the oven and set aside to cool on the cookie sheet for 10-15 minutes. Transfer to a rack. Repeat with another batch or bake two trays at a time.