Prep the Oven and Pan: Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper. This is key, as the high moisture content of the bananas can cause sticking.

Cream Wet Ingredients: In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a hand mixer), cream the softened butter, granulated sugar, and brown sugar together on medium speed until the mixture is visibly lighter, fluffy, and smooth—about 3 to 4 minutes.

Add Eggs and Banana: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and the mashed ripe bananas. The mixture will look slightly separated or 'curdled' due to the moisture, but don’t worry, the dry ingredients will bring it together.

Whisk Dry Ingredients: In a separate, medium bowl, whisk together the flour, rolled oats, baking soda, salt, and cinnamon. Whisk until the leavening agent (baking soda) is evenly distributed. This ensures the easy chunky monkey cookies with bananas rise properly.

Combine: Slowly add the dry mixture to the wet mixture, mixing on low speed until *just* combined. Stop immediately when no streaks of dry flour remain. Overmixing will develop too much gluten, resulting in a tougher cookie.

Fold in the 'Chunky Monkey': Using a spatula, gently fold in the chocolate chunks and the chopped walnuts (or pecans). The batter will be very thick and moist—more like a bread batter than a typical cookie dough.

Scoop and Bake: Drop the dough onto the prepared baking sheets using a standard ice cream scoop or a 3-tablespoon measuring spoon. Leave about 2 inches between cookies. They do not spread dramatically, thankfully resulting in that lovely, chunky profile.

Bake Time: Bake for 12 to 15 minutes, or until the edges are nicely set and just beginning to turn golden brown, and the centers look slightly soft. Remember, the key to a soft chunky monkey banana cookies texture is underbaking slightly.

Cool: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. This is crucial for their texture and prevents them from breaking.