Position a rack in the middle of the oven. Preheat oven to 350 degrees. Have 2 baking sheets ready.
In a bowl, stir the flour, baking soda and salt. Sift the cocoa powder onto the mixture and stir to combine. Set aside.
With an electric mixer on medium speed, beat the butter, sugars, water and vanilla until smooth, about 30 seconds. Add the flour mixture, mixing just until it is incorporated. Stir in the chocolate chips.
Use a thin metal spatula to spread half the dough on one baking sheet into a rectangle that measures about 11-by-8 inches and about ¼ inch thick. Leave a 3-inch border on all sides. Use your fingers or the spatula to pat it into an even layer. Repeat with the remaining dough on the second baking sheet.
Bake one sheet at a time until the top looks dull, not shiny, and feels evenly firm at the edges and center, about 14 to 16 minutes. As soon as each baking sheet comes out of the oven, sprinkle half of the chocolate chips over each cookie slab. Let the chocolate chips sit for 5 minutes, to soften and melt. Use a small metal spatula to spread the melted chocolate over the cookie, covering most of the cookie. While the chocolate is still warm, sprinkle half the peppermint candy evenly over the slab.
Let the cookie bark cool on the baking sheets on a wire rack until the chocolate topping is firm, about 2 hours. Or to speed the cooling, let the cookies sit for about 30 minutes and then refrigerate them, still on the baking sheets, just until the chocolate topping firms. Remove from the refrigerator. The cookie bark will become crisp as it cools.
Break each cookie slab into irregular pieces. The cookies can be stored layered between sheets of wax paper in a tightly covered container at room temperature for up to 5 days. (Text copyright 2005 by Elinor Klivans ⁄ published by Chronicle Books).
You can scoop and drop them by tablespoonfuls onto baking sheet lined with parchment paper or a nonstick liner. Or fill a pastry bag fitted with a ½-inch (no. 6 or 7) plain tip and pipe the meringue onto the baking sheet, forming "kisses" about 1-inch in diameter. Space them 1-½ inches apart.
From Big Fat Cookies by Elinor Klivans, text copyright 2005 by Elinor Klivans ⁄ published by Chronicle Books.