Preheat your oven to 350°F and line a baking sheet with parchment paper.

Melt the butter and set it aside to cool slightly.

In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add the egg and mix until a creamy mixture forms.

Add the flour, baking soda, and salt, and mix until a smooth cookie dough forms.

Fold in the white chocolate chips and Lucky Charms cereal until evenly distributed throughout the dough.

Using a standard cookie scoop, portion the dough onto the prepared baking sheet, spacing the cookies about 2 inches apart. You should get approximately 12 cookies.

Bake for 9 to 11 minutes, or until the edges are lightly golden and the centers still appear slightly soft.

Remove the baking sheet from the oven and immediately press additional Lucky Charms marshmallows onto the tops of the warm cookies.

Allow the cookies to cool on the baking sheet for 5 to 10 minutes before transferring them to a wire rack to cool completely.