Preheat the oven to 350°F. Spray a 9x9-inch square baking dish with nonstick cooking spray, or line with parchment paper for easy removal and cutting. Set aside.
In a large mixing bowl using an electric hand mixer, or use a stand mixer with the paddle attachment, beat the butter, sugar, and light brown sugar until it's thoroughly combined and creamy. About 1-2 minutes.
6 tablespoons butter, 1/3 cup granulated sugar, 1/3 cup packed light brown sugar
Add in the mashed bananas, yogurt, egg, and vanilla extract. Beat until combined well.
1 cup mashed ripe banana, 1/3 cup vanilla yogurt, 1 large egg, 1 teaspoon vanilla extract
Add in the flour, baking powder, salt, milk chocolate chips, and beat on low speed just until combined and no flour streaks remain in the batter.
2 cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon sea salt, 1 cup milk chocolate chips
Evenly spread the batter into the prepared baking dish. Sprinkle some extra milk chocolate chips on top if wanted.
Bake for 25-30 minutes. A toothpick inserted into the center should come out clean, the bars will be puffy and the edges slightly pulled away from the pan.
Let the bars cool completely, inside the pan on a wire cooling rack, for 1 hour. The bars are yummy eaten slightly warm but they cut much nicer when they are fully cooled before cutting and serving. Enjoy!