Heat the oven to 350°F.
Line a 9x13-inch metal baking pan with parchment paper, leaving extra over the sides.
Lightly spray the parchment paper.
Whisk the melted butter, brown sugar, and granulated sugar until smooth.
Add the eggs and vanilla.
Whisk until the mixture looks glossy.
Stir the flour, cornstarch, baking soda, baking powder, and salt together in a separate bowl.
Fold the dry ingredients into the wet mixture.
Stop mixing once no dry streaks remain.
Fold in the red, white, and blue chocolate candies, white chocolate chips, and patriotic sprinkles.
Press the dough evenly into the prepared pan.
Add extra candies and sprinkles on top.
Bake for 24 to 29 minutes.
The bars are done when the edges look golden and the top looks set.
A toothpick should show moist crumbs, not raw batter.
Cool the bars fully before slicing into squares.