How to Make It
Preheat oven to 350°F. Whisk together butter, both sugars, eggs, and vanilla in a large bowl. Whisk together flour, baking powder, and salt in a medium bowl. Add flour mixture to butter mixture, and stir gently just until combined. Fold 1 cup of the candies into batter. Pour batter into a lightly greased 13- x 9-inch baking pan. Scatter sprinkles over the top.
Bake in preheated oven 15 minutes. Remove from oven, and scatter remaining 1/2 cup candies on top. Return to oven, and bake until a toothpick inserted in the middle comes out clean, about 20 more minutes. Let cool completely on a wire rack, about 1 hour.
Melt white chocolate in a microwavable bowl on MEDIUM power until melted and smooth, about 90 seconds, stirring every 30 seconds. Transfer to a piping bag or ziplock plastic bag with a corner snipped. Pipe a zigzag of white chocolate over cooled cake. Let stand 10 minutes before cutting into squares.