Preheat oven to 350°F. Line a 9×13-inch pan with parchment, leaving overhang; lightly coat with cooking spray.
Whisk melted butter with brown sugar and granulated sugar until glossy and combined.
Beat in the beaten eggs and vanilla until the mixture looks smooth and slightly thickened.
In a bowl, whisk flour with baking powder and salt to combine.
Stir dry ingredients into wet just until no dry streaks remain; do not overmix.
Fold in candies, white chocolate chips, and most of the sprinkles, reserving a little for topping.
Spread batter evenly in the pan. Smooth the top and scatter remaining candies and sprinkles over it.
Bake until edges are lightly golden and a tester comes out with moist crumbs, about 20–24 minutes.
Cool completely in the pan. Lift out with parchment and slice into bars.