To make the dough combine flour, butter, sugar and salt in a food processor and pulse until the mixture resembles a coarse meal. With the machine running, slowly add ice water just until dough forms. Cover and chill for 30 minutes.
On a lightly floured surface, roll out dough into an 11-inch round. Transfer the dough to a baking sheet.
In a bowl combine cranberries, walnuts, sugar and dark brown sugar and toss together. Place cranberry mixture in the tart shell, leaving a 1 1/2-inch border around the outside edge. Raise the borders of dough to enclose the sides of the tart, draping it over the fruit. Gently press the edge of the folded dough onto the baking sheet, being careful not to mash the fruit. Pinch the pleats that form from the draping.
Bake the tart for 15 to 18 minutes or until the crust is golden brown and the berries are juicy. Cool the crostata on a rack for 10 minutes and dust with confectioner's sugar.