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.
Melt chocolate gently for best results. If chocolate gets too hot, it may not set properly and will develop "bloom" (white streaks) on the surface when stored. Stirring chopped chocolate in a pan or bowl over hot, not simmering, water maintains an even, low temperature, resulting in glossy, firmly set chocolate. The setting time for the bark varies greatly according to the temperature of your home. Although we preferred the texture of the bark set at room temperature, you can set bark in the refrigerator if necessary.