In the bowl of a mixer or in a large bowl with a hand mixer, cream butter and sugar on medium speed until light and fluffy, 3-4 minutes.
Add vanilla and salt and blend until combined. Stir in Lucky Charms marshmallows.
With mixer on low, slowly add flour to bowl until well combined. Keep mixer on low and add milk in until dough comes together.
Remove pan with cereal crust from freezer and press cookie dough into an even layer over crust.
Spread buttercream frosting over the top of the cookie dough and smooth evenly with a spatula. Press additional marshmallows onto the top of the frosting.
Place pan in fridge chill for at least 45 minutes. Once chilled, remove bars from pan by lifting foil. Using a sharp knife, slice into squares. You can cut large squares and get about 16 or small squares and get about 36.