Preheat the oven to 350 degrees Fahrenheit. Grease a 9x9 baking pan on the bottom and the sides.
Combine the flour and sugar in a small bowl. Using a pastry blender, cut in the butter until you have coarse crumbs.
Pat the shortbread mixture into the bottom of the prepared pan.
Bake for 15-18 minutes.
While the crust is baking, combine the egg, sugar, juice, flour, baking powder and lemon zest in a bowl. Beat until smooth.
Pour the mixture over the warm crust.
Bake for 25 minutes.
Cool the bars and then sprinkle them with sifted powdered sugar.
Cut into 12 squares and store the bars in an airtight container.
The calories shown are based on the bars being cut into 12 pieces, with 1 serving being 1 bar. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.