Heat the oven to 350°F. Whisk the flour, baking soda, matcha powder, and salt together in a medium bowl and set aside.
In a large bowl, cream the butter and brown sugar together on medium speed until creamy, 1 to 1 1/2 minutes.
Add the egg and vanilla and beat to combine, using a rubber spatula to scrape the sides and bottom of the bowl as necessary. Mix in the mashed banana and 1/4 cup of water, beating until incorporated.
Add the dry ingredients and beat on low speed until they’re just combined. Add the oats and the chocolate chips, mix briefly, and use a rubber spatula to scrape the sides and bottom of the bowl if needed.
Line a rimmed baking sheet with aluminum foil and coat lightly with nonstick cooking spray. Use a tablespoon to scoop out 1 1/2-inch dough balls, placing them about 1 inch apart on the baking sheet.
Bake until just golden on top, about 10 minutes. Cool on the sheet for 10 minutes before transferring the cookies to a wire rack to cool completely.
