Preheat the oven to 350˚ F. Line a 9-by-13-inch baking dish with foil, leaving an overhang on all sides. Lightly coat with cooking spray. Whisk the flour, salt and baking powder in a medium bowl.
Beat the butter and sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Beat in the egg, then beat in the food coloring and vanilla and peppermint extracts, scraping down the bowl as needed. Beat in the flour mixture in two batches until just combined.
Spread the dough in the baking dish, pressing it with your hands until flat and even. Bake until set and a toothpick inserted into the center comes out clean, 30 to 40 minutes.
As soon as the bars come out of the oven, top with the white chocolate chips in a single layer. Let sit 2 minutes to soften, then spread into a thin layer with an offset spatula. Put the red candy melts in a small microwave-safe bowl and microwave in 30-second intervals, stirring, until melted. Transfer to a small resealable plastic bag and snip a corner. Pipe diagonal stripes on top of the white chocolate. Sprinkle with the peppermint candies. Transfer the pan to a rack and let the bars cool completely.
If the white chocolate is still soft, place the baking dish in the freezer for a few minutes until set. Lift the bars out of the baking dish, peel off the foil and cut into pieces.