Brown the butter: In a medium saucepan, melt butter over medium heat, stirring constantly until it turns golden brown and smells nutty (about 5–7 minutes). Remove from heat and let it cool slightly.
Mix the wet ingredients: In a large bowl, whisk together the browned butter, brown sugar, and granulated sugar until smooth. Add eggs and vanilla, mixing until combined.
Combine the dry ingredients: In another bowl, whisk together flour, baking soda, salt, and cinnamon. Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Fold in the add-ins: Gently fold in the chocolate chips and marshmallows to create gooey s’mores cookies.
Scoop the dough: Using a cookie scoop or tablespoon, drop rounded balls of dough onto a parchment-lined baking sheet, leaving 2 inches apart.
Bake: Bake at 350°F (175°C) for 9–11 minutes, or until the edges are golden brown and the centers look just set. For extra gooey centers, avoid overbaking.
Cool and enjoy: Allow cookies to cool on the baking sheet for 2–3 minutes before transferring to a wire rack. Enjoy the melty chocolate and marshmallow goodness in these chewy, soft brown butter smores cookies!