Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats. In a medium bowl, whisk together the all-purpose flour, baking soda, and salt until evenly combined; set aside.

Cream butter and sugars

In a large mixing bowl or bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and light brown sugar on medium speed until light and fluffy, about 2–3 minutes. Scrape down the bowl as needed.

Add eggs and flavorings

Beat in the eggs one at a time, allowing each to incorporate fully. Add the vanilla extract and mashed banana and mix on low speed until combined. If using instant mashed banana flakes, sprinkle them in now and mix briefly to hydrate.

Combine wet and dry

With the mixer on low, gradually add the flour mixture to the wet ingredients in two additions, mixing just until no streaks of flour remain. Do not overmix; the dough should be thick and slightly sticky.

Mix-ins & Finishes

Fold in mix-ins

Using a rubber spatula or wooden spoon, gently fold the semisweet chocolate chips, peanut butter chips (or chopped peanuts), and chopped walnuts (if using) into the dough until evenly distributed.

Portion and bake

Scoop dough by rounded tablespoons or a 1.5-tablespoon cookie scoop onto the prepared baking sheets, spacing cookies about 2 inches apart to allow spreading. For larger cookies, use a 2-tablespoon scoop and increase baking time slightly. If you like a slightly domed top, chill scooped dough in the fridge for 15–20 minutes before baking.

Bake

Bake in the preheated 350°F (175°C) oven for 10–12 minutes, or until the edges are set and lightly golden but centers remain soft. Rotate sheets halfway through baking for even color.

Finish

Remove cookies from oven and immediately sprinkle a small pinch of flaky sea salt on each warm cookie. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.