Preheat the oven to 350°F/180°C. Line a square 8×8 baking pan with parchment paper and set it aside.
In a large bowl, mash the ripe bananas with a fork. It's okay if there's chunks of banana. Add the melted butter and whisk until combined.
3 medium bananas, 1/3 cup butter
Add in the dark brown sugar, egg, egg yolk, and vanilla extract. Continue whisking until smooth. There will be clumps because of the banana, that is ok.
1/2 cup dark brown sugar, 1 egg, 1 egg yolk, 1 tbsp vanilla
Add in the flour, baking soda, baking powder, cocoa powder, cornstarch, and salt. Using a spatula, gently fold until a chocolate batter forms.
1 cup all-purpose flour, 1/2 cup unsweetened cocoa powder, 1 tsp baking soda, 1/4 tsp baking powder, 1 tsp cornstarch, 1/4 tsp salt
Add in the chocolate chips and chocolate bar and mix in a few times until well incorporated. Don't overmix!
3/4 cups semi-sweet chocolate chips, 1 milk chocolate bar
Transfer the batter into your prepared pan and spread evenly. Add extra chocolate chips and chopped chocolate on top if desired (optional).
Bake for 50-60 minutes. A toothpick inserted in the middle will come out mostly clean, with some chocolate on it from the chocolate chips. If it it wet with batter, the squares are not ready yet, so continue baking.
Remove from the oven and allow to cool on your kitchen counter either in the pan or on a cooling wire rack. For easy removal of the bars, run a butterknife around the edges of the pan before lifting it up with the parchment paper.
Serve at room temperature, or warm, and enjoy!