Prepare pan: spray 13x9x2-inch metal or glass baking pan with nonstick spray. Line bottom of pan with parchment paper, leaving overhang on both sides of pan.
Put candy canes in a plastic gallon-sized bag and whack with the back of a heavy metal measuring cup (or any other metal tool) until candy canes are smashed but still in pieces (not dust).
Whisk flour and salt in medium bowl.
With an electric mixer, beat butter and sugar on medium speed. 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 40 minutes. Place pan on rack; immediately sprinkle bittersweet chocolate over.
Return pan to oven for one minute.
Using offset spatula (recommended) or the back of a spoon, spread the chocolate over top of cookie in thin even layer. Immediately sprinkle chopped peppermint candies over.
Chill until 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.