Line a 13x9-inch baking pan with foil, extending foil over edges. Lightly spray foil with nonstick cooking spray.
In a very large pot or Dutch oven melt butter over low heat. Add marshmallows; stir constantly and cook until melted. Remove pot from heat and stir in marshmallow fluff and vanilla. Gently stir in Lucky Charms and jimmies until cereal is evenly coated. Pour mixture into prepared pan and use wax paper over the top to press mixture firmly into the pan without it sticking to your hands. Let treats cool for at least 1 hour before frosting.
FOR THE BUTTERCREAM:
Using a stand mixer beat butter on medium-high speed until smooth, about 30-60 seconds. Reduce speed to medium-low and slowly add confectioners’ sugar, and beat until smooth, about 2-5 minutes. Add vanilla, cream, and salt and increase speed to medium-high and beat until light and fluffy, about 4-8 minutes. Spread buttercream evenly over treats (I like using a small offset spatula).
Top with Lucky Charms marshmallows and rainbow jimmies. Using the foil liner, lift treats from pan. Cut into 24 bars and serve.
This recipe makes a TON--24 hefty-sized treats. You can easily halve the recipe.
These Lucky Charms Treats would also be great for potlucks and bake sales because they make a TON!
These will keep in an airtight container at room temperature for about 3 days. After 3 days, they start getting a little stale and hard.