Preheat oven to 350°F (180°C). Grease and line 8-inch (20 cm) square baking pan with enough parchment paper to overhang edges.
Melt butter in saucepan set over medium heat; stir in sugar. Transfer to large mixing bowl; let cool completely.
Using electric mixer, in a bowl, beat egg into sugar mixture; stir in vanilla. Whisk together flour, baking powder and salt; stir into sugar mixture.
Fold in 1/2 cup (125 mL) Cocoa PEBBLES™ Cereal. Scrape batter into prepared baking pan; smooth top. Sprinkle with remaining cereal.
Bake for 20 to 25 minutes or until tester inserted in center comes out with just a few moist crumbs clinging to it or when tester inserted about 2 inches (5 cm) from edge of pan comes out clean. Let cool completely on rack.
Remove from pan; cut into bars or squares.
For frosted blondies, top with store-bought cream cheese frosting and sprinkle with additional Cocoa PEBBLES™ Cereal.