Cut a sheet of wax paper and press it flat into your 10×15-inch pan. Smooth out big wrinkles so the bottom of the bark stays neat. Set the pan on a level spot in your kitchen, ready to go.
Chop the pistachios into small pieces and measure the cranberries into a bowl. Keep any extras, like pretzels, within easy reach. Melted chocolate sets fast, so having toppings ready makes a big difference.
Place the chocolate in a microwave-safe bowl. If you use chips, stir in the shortening or oil first. Heat at 50% power for 30 seconds, then stir, and repeat until the chocolate looks smooth.
Pour the melted chocolate onto the lined pan. Use a spatula to spread it into an even layer about ¼ inch thick. Don't stress about perfect edges, because rustic shapes look charming.
Scatter the cranberries and pistachios over the warm chocolate while it is still soft. Press them down lightly so they stick. Cover the surface well, since bare spots look dull after setting.
Leave the pan on the counter until the chocolate firms up. After it sets, you can move it to the fridge to speed things along. It usually takes about an hour to become fully hard.
Lift the bark off the wax paper and snap it into uneven pieces. Big shards look dramatic, while small bites suit parties. Arrange them on a plate and watch them vanish.