Preheat your oven to 325 degrees Fahrenheit. Line a baking sheet with parchment paper and set aside.
In a large mixing bowl or in the bowl of a stand mixer, cream together the softened butter, brown sugar, and granulated sugar until light and creamy.
Add the egg and vanilla into the cookie dough and beat again until just combined.
Add the flour, baking soda, and salt and beat until just combined. Do not overmix.
Use a wooden spoon or rubber spatula to mix in the white chocolate chips and the Lucky Charms marshmallows until evenly distributed.
Scoop the cookie dough onto a parchment-paper lined cookie sheet using a #24 cookie scoop (about 2 ½ Tablespoons each).
Bake in the preheated oven for 12–15 minutes or until the edges turn brown but the centers are still soft.
Allow the cookies to cool on the baking sheet for a few minutes. After 5 minutes, transfer the cookies to a wire rack to cool the rest of the way.
Serve and enjoy!