Whisk together the melted butter and brown sugar in a large bowl. Then add the egg and vanilla extract and whisk to combine.
Add the flour, baking soda, and salt. Mix just until no large lumps of flour remain.
Next, add the chocolate chips and marshmallows. Gently fold them into the cookie dough, making sure not to rip the marshmallows.
Cover the cookie dough and place it in the fridge to chill for 30 minutes (or up to overnight). While the dough is chilling, preheat the oven to 350 degrees F and line two large baking sheets with parchment paper.
When the dough is chilled and solid, scoop out 2-tablespoon-sized portions and place them at least 2” apart on the prepared baking sheet.
Bake the cookies for 12-14 minutes, until they are barely golden brown around the edges.
Carefully tap the baking sheet on the counter a few times to sink the cookies and then allow them to cool completely on the tray.
Notes
Only mix the butter and dry ingredients until there are no large lumps of flour left.
Fold the marshmallows in carefully so they don’t rip.
For even cookies, use a cookie scoop to scoop the dough onto the baking sheet.
Chilling the dough briefly before the cookies bake keeps the marshmallows from melting too quickly in the oven.
Tap the cookie tray on the counter when the cookies come out of the oven to sink the centers for a perfect, chewy texture.
Store leftovers in an airtight container on the counter for up to 5 days.
This nutrition information is based on the exact products I used in this recipe. Brands and sizes of products could alter exact nutrition and should always be calculated independently.