Preheat the oven to 350F. 

Lightly grease an 8" x 8" square baking pan.

Add the mashed banana, brown sugar, milk, and vanilla extract to a large bowl and mix well. 

Stir in the flour, cinnamon, baking soda, and salt. 

Fold in ¼ cup of the chocolate chips (setting aside the remaining ¼ cup to use in the next step).
OPTIONAL: Fold in half of the walnuts (setting aside remaining).

Pour the batter into the baking pan and spread evenly in the pan. It will be fairly thick. 

Sprinkle over the remaining chocolate chips (and walnuts).

Bake 15 to 20 minutes until a toothpick inserted in the center comes out clean. Let cool for at least 10 minutes before slicing. 

Cut into 9 bars and lift them from the pan using a spatula. Enjoy still warm or let cool completely then store in an air-tight container.