Rochers, French for "rock," may look like little boulders, but their texture is decidedly light and crisp.
Preheat oven to 325 degrees. Mix pistachios, cranberries, and confectioners' sugar.
Combine whites, granulated sugar, and salt in a mixer bowl. Place over a pot of simmering water, and whisk until sugar dissolves and mixture is warm to the touch, about 2 minutes. Attach bowl to mixer, and whisk on medium speed until foamy. Add cream of tartar, and beat on medium-high until stiff, glossy peaks form, about 6 minutes. Fold in pistachio mixture.
Transfer mixture to a pastry bag fitted with a plain 1/2-inch round tip. Line 2 baking sheets with parchment. Pipe mounds 1 inch wide and 1 1/2 inches high with tapered tops (cookies should resemble miniature Christmas trees) onto parchment-lined baking sheets. Reduce oven temperature to 200 degrees. Bake until cookies can be lifted from parchment, about 1 hour and 15 minutes. Let cool on sheets. Dust with confectioners' sugar just before serving.
Rochers will keep, covered, for up to 3 days.