In a medium saucepan over medium heat, melt the unsalted butter. Stir frequently for about 5–7 minutes until the butter turns a deep golden brown and develops a nutty aroma.

Remove from heat and let your browned butter cool slightly for about 10 minutes before mixing it into your dough.

In a large mixing bowl, combine the cooled brown butter with granulated sugar and brown sugar. Beat until fluffy and well-blended, about 2–3 minutes.

Add in the eggs one at a time, ensuring each is fully incorporated before the next addition.

In a separate bowl, whisk together the all-purpose flour, baking soda, and kosher salt.

Gradually add the dry ingredients into the wet mixture, stirring gently until combined.

Carefully fold in marshmallows and chocolate chips until evenly distributed.

Cover the cookie dough and chill in the refrigerator for at least 30 minutes.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop generous tablespoons of dough onto sheets.

Bake for 10–12 minutes until edges are golden and centers still appear slightly underbaked.

Allow cookies to cool on the baking sheets for about 5 minutes before transferring to a wire rack to cool completely.