01 - Preheat the 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. Set aside.
03 - In a large bowl, combine melted butter, brown sugar, and granulated sugar. Mix until smooth.
04 - Beat in the egg, then the egg yolk, and add vanilla extract. Mix well.
05 - Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
06 - Gently fold in the white chocolate chips and 1 cup of the Lucky Charms marshmallows, reserving 1/4 cup for topping.
07 - Scoop dough using a tablespoon or small ice cream scoop onto prepared baking sheets, spacing cookies about 2 inches apart.
08 - Lightly press a few reserved marshmallows onto the top of each dough ball for a festive look.
09 - Bake for 9 to 11 minutes, until the edges are set and the centers look slightly underbaked.
10 - Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.