Combine dry ingredients (flour, sugar, cocoa powder, baking powder, and salt) in a mixing bowl.
Beat together the mashed avocado, milk, water, and vanilla extract in another bowl. Add the dry ingredients, mixing until well combined. Optional: gently combine mixed walnuts.
Grease a 9×13″ baking pan, and pour in mixture.
Bake for 25-35 minutes, or until toothpick comes out clean. Let brownies cool completely before cutting into squares and serving. Enjoy!