Preheat the oven to 350°F. Spray 13x9x2-inch rimmed metal baking pan with nonstick spray. Line the bottom of the pan with a long strip of 9-inch-wide parchment paper, leaving an overhang on both short sides of the pan.
In a medium bowl, whisk the flour and salt together; set aside. In a large bowl, use an electric mixer to beat the butter until creamy, about 2 minutes. Gradually beat in the sugar. Continue beating until the mixture is light and fluffy, stopping occasionally to scrape down sides of bowl, about 3 minutes. Beat in the vanilla, then the egg yolk. Gradually add in the flour mixture, beating on low speed just to blend.
Drop the dough by tablespoonfuls into the prepared baking pan, spacing evenly. Using moistened fingertips, press the dough to form even layer over the bottom of the pan. Pierce the dough all over with a fork.
Bake the cookie base until it is light golden brown, slightly puffed and the edges begin to come away from the sides of the pan, about 30 minutes. Place the pan on a rack; immediately sprinkle the bittersweet chocolate evenly over the hot, baked cookie. Let it stand until the chocolate softens, about 3 minutes. Using small offset spatula, spread the bittersweet chocolate over the top of the cookie in a thin, even layer. Immediately sprinkle chopped candy cane kisses over the chocolate (or crushed candy canes).
In a medium metal bowl set over saucepan of simmering water, stir the white chocolate until melted and smooth. If it's too tough and thick to stir, add a tiny drizzle of vegetable oil or 1/2 teaspoon of shortening to thin it out. Remove the bowl from over the water. Using a fork, drizzle the white chocolate all over the cookies. Chill until the white chocolate is set, about 30 minutes.
Using the paper overhang as an aid, lift the cookie from the pan and transfer it to a work surface. Using a large knife, cut the cookie into irregular pieces.
Notes
*MAKE AHEAD: These can be made 1 week ahead. Store in the refrigerator (or freezer) in airtight containers between layers of waxed paper or parchment paper. Can be stored in the freezer for several weeks.