Once you've sampled this melt-in-your-mouth confection, you'll be tempted to fill your own candy dish with it instead of giving it away as gifts. Best to plan on making a couple of batches from the start. Just enlist a young helper--swirling the white and semisweet chocolates is a particularly popular job--and package the finished candy in see-through cellophane bags to show off the mouthwatering goods.
Heat the oven to 350º and lightly toast the pistachio nuts on a baking sheet, about 10 minutes, stirring occasionally. Let the nuts cool.
Melt the semisweet chocolate in the top of a double boiler set over simmering water, stirring until smooth. Or microwave it, in a dry microwave-safe bowl, uncovered, on medium power for 2 to 3 minutes, stirring once. Remove the chocolate from the microwave and stir until smooth. Melt the white chocolate separately, following the same directions.
In a small bowl, combine the nuts and cranberries, then stir half of them into the semisweet chocolate. Using a spatula, spread the mixture to about a 1/2-inch thickness on a large cookie sheet. Drop the white chocolate by tablespoonfuls over the dark. With the tip of a butter knife, swirl the chocolates together to create a marbled effect. Sprinkle on the rest of the nuts and berries.
Refrigerate the bark for about 1 hour or until firm, then break it into pieces. Store the bark in an airtight container in the refrigerator for up to a month. Makes about 1 3/4 pounds.