Heat oven to 350 degrees. Butter an 8-by-8- by-2-inch baking pan, and set aside.

In a large bowl, combine 1 cup flour, butter, and granulated sugar. Using a handheld electric mixer on medium speed, blend until crumbly and combined. Transfer mixture to prepared pan. Using your hands, pat crust evenly into pan. Bake until golden brown, about 25 minutes. Transfer to a rack to cool.

Combine remaining 1/3 cup flour, apricot jam, eggs, walnuts, baking powder, vanilla, and salt. Using a handheld electric mixer on medium speed, beat until combined; scrape down sides of bowl twice. Pour mixture over crust, and return to oven. Bake until top is golden, about 20 minutes.

Transfer pan to a wire rack to cool. Cut into 2-inch squares. Dust with confectioners sugar.