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.