1. Preheat the oven to 325°F. Line a rimmed baking sheet with aluminum foil and coat lightly with cooking spray.
2. In a medium bowl, whisk together the baking powder and salt; set aside.
3. In a large bowl, whisk together the coconut oil, vanilla, and sugar (or stevia) to combine. Add the egg (or water) and whisk well to incorporate.
4. Using a rubber spatula, scrape down the sides and bottom of the bowl as necessary. Mix in the mashed banana and stir until well incorporated.
5. Add the dry ingredients to the wet ingredients and stir until combined. Using a rubber spatula, fold in the oats and chocolate chips. Sprinkle in the sifted matcha powder and fold in gently.
6. Chill the cookie dough in the refrigerator for 15 minutes.
7. Using a tablespoon, scoop out 11/2-inch balls and place them about 1 inch apart on the prepared baking sheet. Bake until lightly golden on top, 10 to 13 minutes. Cool on the baking sheet for 5 minutes before transferring the cookies to a wire rack to cool completely.