Preheat oven to 350 degrees F and line 8 x 8 square baking dish with unbleached parchment paper. Set aside.
In a large bowl, add bananas and mash with a fork. Measure to make sure you have exact 1 1/3 cups.
Add eggs, peanut butter, milk, cacao powder, whey protein powder and salt. Mix with spatula until smooth. Add walnuts and mix a few times to incorporate.
Pour batter into prepared baking dish, level with spatula and sprinkle with mini chocolate chips. Bake for 25 minutes or until toothpick inserted in the center comes out almost clean, a bit wet and with small pieces of dough attached to it is fine. You will see the edges look more firm too.
Remove from the oven and let brownies cool for 10 minutes. Holding onto parchment paper flaps, transfer to a cooling rack and let cool for an hour or until room temperature before slicing. Cut into 9 squares.
Notes
Store: Store in a ventilated container for 2 days in a cool dark place. Then refrigerate for up to 3 more days.
Freeze: Freeze in an airtight container for up to 3 months.