Preheat oven to 350°F Spray 13x9x2-inch metal baking pan with nonstick spray. Line bottom of pan with long strip of 9-inch-wide parchment paper, leaving overhang on both short sides of pan. Whisk flour and salt in medium bowl. Using electric mixer, beat butter in large bowl until creamy, about 2 minutes. Gradually beat in sugar. Continue beating until mixture is light and fluffy, stopping occasionally to scrape down sides of bowl, about 3 minutes. Beat in vanilla, then egg yolk. Gradually add flour mixture, beating on low speed just to blend.
Drop dough by tablespoonfuls into prepared baking pan, spacing evenly. Using moistened fingertips, press dough to form even layer over bottom of pan. Pierce dough all over with fork.
Bake cookie base until light golden brown and slightly puffed and edges begin to come away from sides of pan, about 30 minutes. Place pan on rack; immediately sprinkle bittersweet chocolate over. Let stand until chocolate softens, about 3 minutes. Using small offset spatula, spread bittersweet chocolate over top of cookie in thin even layer. Immediately sprinkle chopped peppermint candies over.
Stir white chocolate in medium metal bowl set over saucepan of simmering water until melted and smooth. Remove from over water. Using fork, drizzle white chocolate all over cookies. Chill until white chocolate is set, about 30 minutes.
Using paper overhang as aid, lift cookie from pan and transfer to work surface. Using large knife, cut cookie into irregular pieces. DO AHEAD Can be made 1 week ahead. Store in refrigerator in airtight containers between layers of waxed paper or parchment paper.