Preheat oven to 350ºF.

Grease a 9" x 13" baking pan and line the bottom of the pan with parchment paper, leaving overhang on both short sides of pan.

In a medium-size bowl whisk together flour and salt. Set aside.

Using a stand mixer, using the paddle attachment mix butter and sugar together until creamed (light yellow in color).

Mix in vanilla extract and egg yolk.

Gradually add flour mixture, beating on low speed just to blend.
Drop tablespoonfuls of dough into prepared baking pan, spacing evenly.

Using moistened fingertips, press dough to form an even layer over bottom of pan. Pierce dough all over with fork.

Bake cookie base until light golden brown, slightly puffed and edges begin to come away from sides of pan, about 30 minutes.

Place pan on cooling rack.

Immediately sprinkle chocolate chips over the hot cookie base.
Let it stand until chocolate softens, about 3 minutes. Using small spatula, spread melting chocolate over top of cookie in thin even layer.

Immediately sprinkle peppermint candies over chocolate layer.

In a double boiler, melt white chocolate chips over low heat until completely melted and smooth. Remove bowl containing white chocolate from the water. Using a fork, drizzle white chocolate all over the cookie. 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.

Cookie bark can be made 1 week ahead. Store in refrigerator in airtight containers between layers of waxed paper or parchment paper.