Preheat oven to 350°. Spray a 9x9-inch baking dish with cooking spray OR line with parchment paper.
In a mixing bowl with a handheld mixer, or bowl of a stand mixer, add the butter, sugar, brown sugar, Greek yogurt, baking powder and salt. Beat until combined well. * At first it will look chunky like it's not mixing together but keep beating it for 1-2 minutes until it's creamy looking and combined.
Add the egg, mashed banana, and vanilla extract. Beat until combined.
Add in the flour and milk chocolate chips and beat on low speed just until combined and no flour spots remain.
Evenly spread the batter into the prepared baking dish. Sprinkle some extra chocolate chips on top (if wanted).
Cook for 22-30 minutes. A toothpick inserted into the middle should come out clean. The bars will be puffy and the edges will have pulled away slightly from the pan. Don't take the bars out if the middle is still wet. They won't continue cooking once you take them out. Make sure a toothpick inserted into the middle does not come out with wet batter on it.
Let the bars cool in the pan for 1 hour before cutting. You can also eat them warm but letting them cool helps the bars cut nicer.
I store leftovers at room temperature in a covered container or a gallon-sized Ziploc bag.
GREEK YOGURT : I like the touch of vanilla with the vanilla Greek yogurt but feel free to also use plain Greek yogurt if you'd rather. TIP : To make spreading the batter easier into the pan drop spoonfuls of it all over the pan and then spread it out. I like to spray the back of a silverware spoon with cooking spray to help push that batter into the pan. This recipe has been updated with better pictures and more helpful text in March 2021. The only change to the recipe was the flour. The original recipe called for 2 cups of white whole wheat flour. I don't think they make it anymore as I can't find it anywhere. The recipe works great with white whole wheat flour but I rewrote the recipe to include all-purpose flour as that is common and what everyone can find. I would advise against using whole wheat flour as that makes the bars really dense and dry. The ratios in the recipe are not meant to be used with whole wheat flour.