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.
Heat the oven to 375°. Grease a 9- by 9-inch baking pan with butter or non-stick cooking spray.
In a mixing bowl, beat the butter and sugar until well blended. Add the flour and salt and beat just until the dough forms clumps. Don't overmix. Turn out the dough onto a piece of waxed paper. Lightly flour your hands and squeeze the dough into a ball (the warmth of your hands will soften the dough enough to hold it together).
Divide the dough into thirds. Crumble one third into a bowl and set it aside. Press the remaining two thirds evenly into the greased pan and bake it on the center oven rack for about 10 minutes, until the edges of the dough are lightly golden. Set the pan on a wire rack. While the dough cools, add the oats and nuts to the reserved dough in the bowl and pinch the ingredients together so that the mixture becomes crumbly.
Spread the raspberry jam on the baked shortbread (it's OK if the dough is still warm), then cover the jam with an even layer of the crumb topping. Bake the bars for 20 to 24 minutes, until the top crumbs are golden brown, then set the pan back on the wire cooling rack. While the shortbread is still slightly warm, cut it into 21/4-inch squares. Once the bars cool completely (they are very fragile while warm), lift them from the pan with a spatula. Makes 16 bars.