Preheat oven to 350° degrees F. Line an 8x8 light metal baking pan with foil or parchment, extending the sides of the foil over the edges of the pan. Spray lightly with cooking spray; set aside.
Meanwhile, combine the cubed butter and chopped white chocolate bars in a large microwaveable bowl. Microwave on HIGH power for 20 seconds. Stir, and microwave for an additional 20 seconds, stopping to stir. Continue microwaving in 20 second increments, stopping to stir after each 20 second burst, until melted and smooth. Let cool for 5 minutes.
To the slightly cooled off white chocolate mixture, whisk in the sugars, then add in the eggs, one at a time, mixing well after each addition, followed by the vanilla. Add in the salt and flour and mix well. In a food processor, add in 1 cup of the Fruity Pebbles cereal and pulse until it becomes a fine crumb consistency. Add in the finely crushed crumbs to the blondie batter, as well as the remaining 1/2 cup whole Fruity Pebbles cereal; mix well.
Spread the batter evenly into the prepared pan. Bake for 25-35 minutes or until a toothpick inserted near the center comes out clean or with moist, but not wet, crumbs. Cool completely before cutting into bars to serve. If desired, sprinkle with fresh Fruity Pebbles cereal before serving.