When you make Peppermint Bark, first line a 12- by 15-inch baking sheet with parchment paper and grease the paper with butter or cooking spray.
Next, place 2 pounds of peppermint candy in a heavy zip-top plastic bag, then place that bag inside another bag for extra strength. Pound the candy with a rolling pin to coarsely crush.
Place coarsely chopped bittersweet chocolate and vegetable shortening in a metal bowl that will nest in a 3- to 4-quart saucepan.
Heat 1 inch of water in the saucepan just until steaming. Remove from heat and place the bowl over the water, making sure that the bowl and water do not touch. Stir occasionally just until mixture melts and is smooth. Remove the bowl from over the pan.
Transfer one and a quarter cups of crushed peppermint candy to a wire mesh strainer; hold it over the melted chocolate and knock the sides to sift fine dust into the chocolate. Reserve large pieces of candy in the strainer.
Stir peppermint dust into the chocolate mixture. Then stir in the remaining unsifted peppermint.
Using a flexible spatula, scrape the mixture onto parchment paper and spread to a quarter to half inch thickness. The mixture should almost cover the baking sheet. Sprinkle with the reserved peppermint from strainer, and gently press into the chocolate.
Let the candy stand at cool room temperature until completely firm, about 4 to 6 hours, or overnight.
Remove the candy from the pan by lifting out the parchment paper., then break or cut the bark into pieces.