Preheat oven: Set temperature to 350°F and grease 8x8 inch baking pan with cooking spray for easy brownie removal.
Mix wet ingredients: Combine ¾ cup peanut butter, ⅓ cup maple syrup, 1 teaspoon vanilla, and 1 cup zucchini until smooth.
Add dry ingredients: Whisk in ¼ cup cacao powder, 2 tablespoons flaxseed, ¾ teaspoon baking soda, and ¼ teaspoon salt.
Fold chocolate chips: Gently stir in ⅓ cup chocolate chips, reserving extra for topping the batter before baking.
Bake brownies: Transfer to prepared pan, sprinkle with chips, and bake 25-30 minutes until knife comes out almost clean.