In a mixing bowl add mashed banana and peanut butter, beat with a mixer just until combined. Add protein powder and chocolate chips, stir to incorporate. Pour mixture into a lightly oiled 8x8-inch casserole pan.
Bake 25-30 minutes. Cool completely before slicing. Tip: dip a butter knife in water before cutting brownies, wipe clean after each slice. Store covered in the refrigerator.