Preheat the oven to 375 degrees Fahrenheit (190 degrees Celsius). Line two baking sheets with parchment paper or silicone baking mats, and set aside.
In the bowl of a stand mixer or a large mixing bowl with a hand mixer, beat the butter until creamy, about 2 minutes on medium speed. Add the sugars and beat for an additional minute.
With mixing speed on low, add the mashed banana, egg, and vanilla extract. Mix until combined.
In a small mixing bowl, whisk together the flour, baking powder, and salt. With mixing speed on low, gradually add the dry ingredients to the wet ingredients. Mix just until combined. Fold in the chocolate chips with a spatula or wooden spoon.
Using a standard-size cookie scoop, scoop balls of soft dough onto the prepared baking sheets. Bake for 10–12 minutes or until the edges begin to brown. The cookies will be soft and muffin-like.
Remove from the oven and let cool for 5 minutes before transferring to a wire rack to cool completely.
Notes
STORE cooled cookies in an airtight container at room temperature for up to a week. To keep them soft, place a slice of bread in the container—the bread will absorb the moisture, preventing the cookies from getting stale.For longer storage, pre-portion the cookie dough into balls, place them in a single layer on a baking sheet and freeze until solid. Transfer the frozen dough balls to a freezer-safe bag with the air removed. The dough can be frozen for up to 3 months. To bake, THAW the dough balls in the refrigerator and bake as directed.