Preheat oven to 350°F. Line a 9x13 baking pan with parchment paper, leaving some overhang for easy removal, and set aside.
In a large mixing bowl, whisk together the melted butter, brown sugar, and sugar until well combined. Whisk in the eggs and vanilla extract until fully combined.
Add the flour, baking powder, and salt and mix until just combined, then fold in the white chocolate chips and 3 Tbsp of sprinkles.
Scoop the batter into the prepared 9x13 pan and spread it around into one even layer and sprinkle remaining sprinkles on top. *The batter is thick. Spray your clean hands with some cooking spray to easily push your batter flat.
Bake at 350°F for 25-30 minutes or until the top is set and lightly browned. Remove from the oven and cool completely before cutting into bars.