Preheat oven to 350℉ (180℃)
Prepare 2 baking sheets by lining with parchment paper or a silicone baking mat
In a medium bowl, whisk together flour, baking soda, cornstarch and ground cinnamon (sift ingredients together if flour is lumpy). Set aside.
1 ½ cup All-purpose flour, 2 Tablespoons Cornstarch, ½ teaspoon Baking soda, ½ teaspoon Ground cinnamon
Using a stand mixer with the paddle attachment or a hand mixer, begin creaming the butter, brown sugar and granulated sugar until light and fluffy (2 - 4 minutes on medium speed). Scrape the bowl as needed.
½ cup Unsalted butter, ½ cup Light brown sugar, ¼ cup Granulated sugar
Add egg and sour cream to bowl and mix on medium speed until incorporated. Scrape the bowl as needed.
1 Large Egg, 2 Tablespoons Sour cream
Add mashed bananas to bowl and mix on medium speed until incorporated. Scrape the bowl as needed.
½ cup Ripe bananas, mashed well
Add flour mixture and mix on low speed until combined.
Place mixing bowl in refrigerator for approximately 1 hour. This allows the flavors to develop and the cookie dough will firm up.
Scoop 1 ½ tablespoon of the dough using a cookie scoop or spoon.
Place onto prepared baking sheets leaving approximately 2 inches of space between each cookie.
Using your palm or the bottom of a round drinking glass, flatten the cookie slightly.
Bake cookies for 10 - 12 minutes.
Let cookies cool on baking sheets for 5 to 10 minutes, then transfer to a wire rack to cool completely.