Heat oven to 375°F. Coat 13 x 9 x 2-inch baking pan with cooking spray.
Combine flour, oats, sugar, butter and almond extract in food processor. Pulse with off/on motion until mixture is well mixed and crumbly, for about 30 seconds. Press the mixture evenly over the bottom of the prepared baking pan.
Bake in 375°F oven for 10 to 15 minutes or until lightly golden. Remove the pan with crust from the oven. Reduce the oven temperature to 350°F.
Whisk together sugar, eggs, egg whites, lemon rind and juice and vanilla in a medium-size bowl until well blended.
Whisk together flour, baking powder and salt in second bowl. Whisk flour mixture into egg mixture until well blended. Pour evenly over crust in pan.
Bake in 350°F oven 15 minutes. Transfer pan to wire rack to cool.
Cut into stars with 2-inch cookie cutter, or cut into squares. Dust with powdered sugar before serving.
