01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the flour, baking soda, and salt until evenly distributed. Set aside.
03 - In a large bowl, combine melted butter, brown sugar, and granulated sugar. Mix until smooth and no lumps remain.
04 - Beat in the egg, then the egg yolk, followed by the vanilla extract. Mix until fully incorporated.
05 - Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined. Avoid overmixing.
06 - Gently fold in the white chocolate chips and 1 cup of the Lucky Charms marshmallows. Reserve 1/4 cup marshmallows for topping.
07 - Using a tablespoon or small cookie scoop, drop dough balls onto prepared baking sheets, spacing them about 2 inches apart.
08 - Lightly press a few reserved marshmallows onto the top of each dough ball for a decorative finish.
09 - Bake for 9-11 minutes until edges are set and centers appear slightly underbaked. They will firm up as they cool.
10 - Allow cookies to rest on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.