Preheat oven to 350°F. Grease 8-inch square baking pan with enough parchment paper to leave overhang. Whisk together flour, baking powder and salt; set aside.
Melt butter in saucepan set over medium heat. Immediately stir in brown sugar; remove from heat. Transfer to bowl; let cool completely.
Whisk in egg and vanilla. Stir in flour mixture just until combined.
Reserve 24 M&M’S®; set aside. Fold remaining M&M’S® into blondie batter; scrape into prepared baking pan, smoothing top.
Bake on rack set in center of oven for 20 to 25 minutes or until tester inserted in center comes out clean with just a few moist crumbs adhering. Transfer to rack; let cool completely. Remove from pan; remove parchment paper. Cut into 12 bars.
Drizzle caramel sauce over top. Garnish each bar with 2 reserved M&M’S®. Scrape melted chocolate into piping bag fitted with small round tip. Pipe dots of melted chocolate on top of each bar to form eyeballs. Tip: Substitute a tube of store-bought brown or black food coloring gel for melted dark chocolate if desired.