Boterkoek (Dutch Shortbread)

Cream together 1 cup powdered sugar and 1/2 pound butter until light. Stir in 2 teaspoons grated lemon rind, 1 teaspoon lemon juice and 1/2 teaspoon vanilla. Sift and measure 2 cups cake flour. Beat into the sugar-butter mixture and add 1/2 teaspoon salt. Bake in a square pan (9"x9"x2") in a moderate oven, 350 degrees, until golden brown on top. Cut into squares while hot. Cool and Serve.