01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, whisk melted butter and brown sugar until smooth and glossy.
04 - Add egg, egg yolk, and vanilla extract. Whisk until fully incorporated.
05 - Gently fold dry ingredients into wet mixture using a spatula until just combined. Do not overmix.
06 - Fold in white chocolate chips and most of the sprinkles, reserving 1 tablespoon for the top.
07 - Spread batter evenly into prepared pan. Sprinkle remaining sprinkles over the top.
08 - Bake for 23-26 minutes, until edges are set and a toothpick inserted in the center comes out with a few moist crumbs.
09 - Let cool completely in the pan on a wire rack. Lift out using parchment overhang, cut into 16 squares, and serve.