Step 1: Preheat oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, leaving overhang for easy removal.

Step 2: In a large bowl, mash bananas until mostly smooth with some small chunks remaining for texture.

Step 3: Stir melted butter and brown sugar into mashed bananas until well combined and mixture is smooth.

Step 4: Beat in eggs one at a time, then add vanilla extract and milk, mixing until fully incorporated.

Step 5: In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt until no lumps remain.

Step 6: Gradually fold dry ingredients into wet ingredients until just combined, being careful not to overmix.

Step 7: Fold in chocolate chips and walnuts (if using), reserving a few chips for topping.

Step 8: Pour batter into prepared pan, spread evenly, and sprinkle remaining chocolate chips on top.

Step 9: Bake for 32-37 minutes until a toothpick inserted in center comes out with a few moist crumbs.

Step 10: Cool completely in pan before lifting out using parchment overhang and cutting into squares.