Measure 1/2 cup mashed banana (about 2 bananas) into a large bowl. Add 2/3 cup firmly packed brown sugar, 1/4 cup granulated sugar, and 1/4 cup melted unsalted butter. Whisk until the mixture is smooth and any large banana pieces are broken up.

Add 1 teaspoon baking soda and 1/2 teaspoon kosher salt to the bowl and whisk again until combined.

Add 1 1/2 cups all-purpose flour. Switch to a spatula and fold the flour into the wet mixture until just incorporated; do not overmix.

Add 4 ounces chopped dark chocolate (chunks or chips, chopped as listed) and 1/3 cup finely chopped walnuts. Fold them into the dough until evenly distributed.

Cover the bowl with plastic wrap and refrigerate the dough for 1 hour.

When ready to bake, preheat the oven to 350°F and line a baking sheet with parchment paper.

Portion the chilled dough onto the prepared baking sheet, placing about six dough balls per sheet and spacing them about 3 inches apart.

Bake each sheet for 10 to 12 minutes, until the cookie edges are medium golden brown.

Remove the baking sheet from the oven and allow the cookies to cool on the sheet for about 10 minutes (they will deflate slightly as they cool). Repeat baking with the remaining dough.