"For a potluck dessert, we stand by these bar cookies, which combine the buttery flavor of shortbread with the sweet-tart taste of jam. Preparing them can be a fun family project: Kids of all ages can help mix the dough and pat it into the pan, as well as sprinkle on the crumb topping. Although simple, the success of this recipe relies on good ingredients, so be sure to use real butter (not margarine) and a quality jam. This recipe comes from the kitchen of Susan G. Purdy, author of "The Family Baker" and "The Perfect Cake" (Broadway Books), and her daughter, Cassandra Purdy, a caterer and former baker at the Cafe Pongo in Tivoli, New York, where she developed this popular dessert...."