Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving overhang for easy removal.

In a medium saucepan, melt butter over low heat. Stir in granulated sugar, brown sugar, cocoa powder, and salt until smooth.

Transfer to a large bowl. Whisk in vanilla extract. Add eggs one at a time, beating until batter is thick and shiny.

Sift in flour and cornstarch. Fold gently until just combined. Pour into prepared pan and spread evenly.

Bake for 20-22 minutes until edges are set but center has slight jiggle. Cool completely in pan.

For ganache: Heat heavy cream until simmering. Pour over chopped chocolate and corn syrup. Let sit 3 minutes, then stir until smooth.

Spread ganache over cooled brownies. Immediately sprinkle with rainbow candy chips. Refrigerate 1-2 hours until set before slicing.