• Line a 9 x 9-inch baking pan with parchment paper or foil and spray with nonstick cooking spray.
• In a large pan, melt butter over low heat. Set aside 1 1/2 cups marshmallows. Add the rest of the marshmallows to the pan and stir until completely melted. Remove the pan from the heat.
• Gradually add crisped rice cereal; stirring until well coated. Fold in the reserved 1 1/2 cups of marshmallows and sprinkles until thoroughly incorporated.
• Using a buttered spatula or clean hands to firmly press the mixture into the prepared pan. Cool completely.

For the Buttercream

• In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, beat butter on medium-high speed for about 5 minutes.
• With the mixer on low speed, slowly add in powdered sugar, and continue beating until well blended.
• Add in vanilla, and 1 teaspoon of milk. Mix on low speed until incorporated.
• Beat at high speed until frosting is smooth and fluffy; about 4-5 more minutes. If your frosting is too thick, gradually add a little more milk, 1 teaspoon at a time until you reach the desired consistency.
• Spread or pipe frosting onto cooled marshmallow cereal treats and top with festive sprinkles.