In a skillet over medium heat, melt the salted butter and continue to cook, stirring frequently, until it turns a rich golden brown with a nutty aroma, about 3-4 minutes. Remove it from heat and cool slightly while you preheat the oven.

Preheat your oven to 350°F (175°C) and line a 9x9-inch baking pan with parchment paper, ensuring it hangs over the edges for easy removal.

In a large mixing bowl, combine the cooled brown butter with dark brown sugar, maple syrup, apple butter, egg, and vanilla extract. Stir until well blended. Gradually add all-purpose flour, baking soda, ground cinnamon, and salt. Once combined, gently fold in the semi-sweet chocolate chips.

Press half of the prepared cookie dough evenly into the bottom of your lined baking pan. Bake for 10 minutes, until the edges start to set but the center remains soft and slightly underbaked.

Once the base layer is baked, switch your oven to broil mode. Carefully remove the pan and arrange the marshmallows over the top. Broil for 1-3 minutes, watching closely until they're golden and toasted.

Flatten the remaining cookie dough and layer it over the toasted marshmallows. Return the pan to the oven and bake for an additional 10-15 minutes until the tops are golden brown.

Allow the cookie bars to cool in the pan for about 10 minutes. Lift them out using the parchment paper and transfer to a cutting board. Cut into squares and enjoy!