Preheat oven to 350 degrees F. Line an 8x8" baking pan with foil and spray with non-stick cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, mix the butter and brown sugar on medium speed for 2 minutes. Add the egg, vanilla, baking powder, and salt and continue mixing for 1 minute, scraping the sides of the bowl as necessary.
Turn the speed to low and add the flour, mixing until just combined. Stir in the white chocolate chips and 1/2 cup sprinkles until evenly incorporated.
Press the dough into the prepared pan, top with the additional 2 tablespoons of sprinkles, and bake for 20-25 minutes, until the edges are lightly golden brown.
Allow to cool in the pan on a wire rack.
Cut into bars when you're ready to serve.
Store airtight at room temperature for up to 3 days.