Line a 9x9 pan with foil and grease the foil. Set aside

In a large saucepan over low heat, melt butter. Add marshmallows and stir continuously until the marshmallows have melted. Remove from the heat and stir in vanilla extract. Immediately stir in Rice Krispies, stirring until well coated. Scatter ¼ cup of sprinkles over the Rice Krispies and stir just a few times until the sprinkles are dispersed. Do not over-mix or the color from the sprinkles could bleed into the Rice Krispies. Lightly press the mixture into the prepared pan, creating an even layer. Use a piece of waxed paper or buttered fingers to help press the mixture into the pan. Don't press too hard or the treats could get tough. Sprinkle the top with remaining 1 tablespoon of sprinkles. Cool before topping with frosting.

Using a hand mixer or stand mixer, beat butter until smooth. Add powdered sugar, vanilla extract, milk, and salt and beat on low speed. Increase to medium-high speed and beat for 1-2 minutes, until creamy and fluffy. Add more milk, if necessary, to thin. Spread the frosting across the top of the bars or use a piping bag fitted with a star tip to pipe frosting onto the top of each bar. Decorate with remaining sprinkles.