Preheat the oven to 350 degrees and line a baking sheet with parchment paper.
In a medium sized saucepan, melt butter over medium heat.
Continue cooking butter over medium heat until brown bits form on the bottom of the pan and butter has become an amber color.
Pour browned butter into a large mixing bowl and let cool completely.
Once butter has cooled, add in brown sugar, granulated sugar, vanilla extract, eggs, and corn syrup. Whisk until fully combined.
Next, add flour, baking soda, and salt. Mix until combined.
Now, fold in chopped chocolate and marshmallow bits until fully incorporated.
Using a 2-inch cookie scoop, place dough balls on the prepared baking sheet.
Bake for 8-10 minutes or until the edges become slightly golden.
Remove from the oven and let cool on the baking sheet for 2-3 minutes before transferring to a wire cooling rack.