Preheat oven to 350 degrees and lightly oil or line a 8 x 8 or 9 x9 baking pan with parchment paper. Set aside.
Combine flour, cocoa powder, baking soda and salt in a large bowl and whisk to thoroughly incorporate. set aside.
In another bowl combine mashed banana, peanut butter, milk and sugar. Mix well.
Pour banana mixture into flour mixture and stir just to combine.
Fold in walnuts, if using.
Spread batter into prepared baking pan in an even layer. Sprinkle with additional chopped walnuts if desired.
Bake for 25 - 35 minutes or until a toothpick inserted comes out clean.
Cool in pan briefly before removing to continue cooling on wire rack.
Once cool cut into squares and enjoy.
Store in an air tight container in the fridge.