Whisk together the melted butter and brown sugar in a large bowl. 

Next, add the egg and vanilla extract. 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 350F 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– this will help them finish cooking all the way through! 

Store any leftover cookies in an airtight container at room temperature for up to five days.