Line 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 Rice Krispies and jimmies until cereal is evenly coated. Pour mixture into prepared pan and use wax paper over the top to press mixture firmly into pan. 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). Garnish with rainbow jimmies. Using foil liner, lift treats from pan. Cut into 24 bars and serve.