In a large mixing bowl, melt butter.
Stir in brown sugar, white sugar, and vanilla extract. Mix together fervently until combined the mix in the egg.
Add in flour, baking soda, and salt. Mix together until cookie dough texture forms. NOTE: if the batter is too soft or the butter isn’t whisked enough, you may need to add another 2 tbsp - ¼ cup of flour. The mixed batter of wet and dry ingredients batter should be a we sand texture.
Add all of the mix-ins to the bowl except the lucky charm marshmallows. Fold together.
Stir in white chocolate chips and Lucky Charm Marshmallows.
Preheat oven to 350°F and chill the dough for 20-30 minutes.
Line a large baking sheet with parchment paper. Use a 1.5 T cookie scoop to scoop dough onto prepared baking sheets. Gently press down so they’re a thick dish cookie instead of a round ball. Press extra Lucky Charms marshmallows into the top of the scooped cookie dough balls (the colorful marshmallow bits help create a more visually festive cookie).
Place cookies on a baking sheet. They won't expand much while baking (if they're chilled).
Bake for 9-11 minutes. The edges of the cookies will be golden and the center will look slightly doughy. If needed, use a cup placed over ‘shake’ the cookie back into a circle shape if they’ve expanded too much while baking.
Let the cookies cool a bit (if the centers are still unstable) before removing them to a wire cooling rack / cooler surface. Enjoy!