1. Line a 12- by 15-inch baking sheet with cooking parchment; butter parchment. Chop 2 pounds white chocolate and place with 1 tablespoon solid vegetable shortening in a heatproof bowl that will nest in a 3- to 4-quart pan. Heat 1 inch of water in the pan just until steaming. Remove from heat and place bowl over water (bowl shouldn't touch water). Stir occasionally just until mixture is melted and smooth. Remove bowl from over pan.
2. Stir in 2 cups each shelled unsalted raw pistachios and dried cranberries. Using a flexible spatula, scrape mixture onto buttered parchment and spread 3/8 to 1/2 inch thick (mixture should almost cover sheet). Sprinkle with 1 more cup each pistachios and dried cranberries; gently press into white chocolate.
3. Let stand at cool room temperature (see notes) until completely firm, 4 to 6 hours, or overnight. Break or cut bark into pieces. Store airtight in a cool place up to 1 month.
Nutritional analysis per ounce.