Preheat oven to 375*. Coat an 8x8 inch square baking pan with cooking spray. In a medium bowl, combine 2/3 cups flour, powdered sugar and oats. Using a pastry blender or your fingertips, cut butter into flour mixture until the mixture resembles fine crumbs. Press into bottom of prepared pan. Bake for 8 minutes. Remove from oven.
Meanwhile, in a medium bowl, using an electric mixer, beat eggs and sugar on high until light and foamy, about 2 minutes. Add lemon juice, 2 tablespoons flour and baking powder. Mix until smooth. Stir in lemon zest.
Pour topping over crust. Bake until lightly brown and center is set, about 15 minutes. Cool completely. Cut into 16 squares. Dust with confectioners sugar. Refrigerate until ready to serve.