In a large mixing bowl, beat butter with electric mixer for 30 seconds. Add 1/2 cup sugar and the salt; beat until combined. Add egg yolks; beat well. Gradually add flour, beating on low speed until combined. (If necessary, stir in last portion of flour by hand.)
Pat dough evenly onto bottom of an ungreased 15x10x1-inch baking pan. Bake in a 350 degree F oven for 12 to 15 minutes or until lightly browned around the edges. Remove from oven. Dollop preserves evenly on crust; carefully spread to cover.
In a small mixing bowl, beat egg whites with an electric mixer on medium speed until soft peaks form (tips curl). Gradually add 1/2 cup sugar, about 1 tablespoon at a time, beating at high speed until mixture forms stiff peaks. Fold in nuts. Dollop over preserves; spread to cover (if small amount of preserves gets into egg-white mixture, cookies won't be harmed).
Bake for 20 minutes more. Remove from oven; let cool on wire rack for 15 minutes. While still warm, cut into 48 pieces. Cool completely before serving. Makes 48 bars.