Preheat the oven to 350°F. Line an 8-inch by 8-inch pan with parchment paper and set aside.
In a medium bowl, whisk together flour, baking powder and salt. Set aside.
In a large bowl, whisk together melted butter, brown sugar and granulated sugar until combined. Add egg and vanilla and mix well. Add the flour mixture and stir just until combined (careful not to overmix). Fold in white chocolate chips and sprinkles.
Transfer batter to prepared baking pan and spread into an even layer. Bake for 25 to 28 minutes or until a toothpick inserted into the center comes out clean. Cool and cut into squares and serve.
Notes
Double the recipe ingredients and baking in a 9x13 metal pan for 28-30 minutes.Store bars in an airtight container at room temperature for up to 5 days.Freeze for up to 3 months. Wrap in plastic wrap and aluminum foil to prevent freezer burn.