Preheat oven at 180 degree Celsius or 350 degrees Fahrenheit. Line a square (20cm/8inch) baking tin with baking paper.
Place flour and sugar into a mixing bowl and mix to combine. Put the cubed butter on top and combine using a pastry cutter or food processor. Stop combining when the mixture has no visible flour and resembles coarse crumbs.
Evenly press the shortbread mixture onto the bottom of the baking tin and bake for 20 minutes. The bottom and top edges should just be starting to brown.
Whisk together all the ingredients (sugar, flour, eggs, lemon juice, zest) using an electric whisk or by hand until the sugar has dissolved. Let set for around 5 minutes.
Whisk again to ensure the sugar fully dissolves. Pour mixture on top of the shortbread base.
Bake at the same temperature (180 degrees Celsius) for 20 minutes until the custard has set. The custard has set when it doesn't jiggle anymore. If the topping is starting to brown too much, place tinfoil above the baking tin.
Let cool completely before cutting. You should be able to get around 12-16 bars. Dust with icing sugar.
Servings 0
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.