01 - In a large mixing bowl, beat the softened unsalted butter with a hand mixer or stand mixer on medium speed until creamy and pale in color, approximately 2 minutes.
02 - Gradually add the sifted powdered sugar, 1 cup at a time, mixing on low speed after each addition until fully incorporated and no dry pockets remain.
03 - Pour in the whole milk and vanilla extract, then increase the mixer speed to medium-high and beat until the frosting is smooth, fluffy, and light, about 2 to 3 minutes.
04 - Using a spatula, gently fold 3/4 cup of Fruity Pebbles cereal into the finished frosting until the pieces are evenly distributed throughout.
05 - Spread or pipe the frosting onto completely cooled cupcakes or a cooled 9-inch cake layer using an offset spatula or piping bag.
06 - Sprinkle the remaining 1/4 cup of Fruity Pebbles cereal over the frosted surface just before serving to preserve maximum crunch and vibrant color.