Melt butter in a saucepan over medium heat. Stir until golden and nutty.
Remove from heat and cool 10–15 minutes. It should be warm, not hot.
Preheat oven to 350°F.
Whisk butter with brown sugar and granulated sugar until glossy.
Add egg, egg yolk, and vanilla. Mix until smooth.
Stir in flour, baking soda, cornstarch, and salt.
Fold in mini marshmallows combined with lucky charms marshmallows. Reserve a handful of lucky charms marshmallows for topping.
Scoop large 3-tablespoon dough balls onto parchment-lined baking sheets.
Press a few Lucky Charms marshmallows into the tops of the dough balls.
Bake 9 minutes.
Centers should look slightly underdone when you pull them.
Let cookies set on the pan 10 minutes before moving.