01 - Preheat the oven to 350°F and line two baking sheets with parchment paper.
02 - Whisk together the flour, baking soda, and salt in a medium bowl. Set aside.
03 - In a large bowl, combine melted butter, brown sugar, and granulated sugar. Mix until smooth and well incorporated.
04 - Beat in the egg, then the egg yolk. Add vanilla extract and mix until fully combined.
05 - Gradually add the dry ingredients to the wet mixture, stirring until just combined. Do not overmix.
06 - Gently fold in white chocolate chips and 1 cup of the Lucky Charms marshmallows. Reserve 1/4 cup marshmallows for topping.
07 - Sop dough using a tablespoon or small ice cream scoop onto prepared baking sheets, spacing cookies about 2 inches apart.
08 - Lightly press reserved marshmallows onto the top of each dough ball for a festive appearance.
09 - Bake for 9-11 minutes until edges are set and centers appear slightly underbaked for soft texture.
10 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.