To begin, preheat your oven to 350 degrees. Grease a 9x13 baking dish and set it aside.
In a mixing bowl, combine the butter, powdered sugar, and flour. Mix until it comes together as a smooth dough. Press the dough into the bottom of the baking dish. Bake for 20 minutes.
Meanwhile, beat the eggs in a mixing bowl. Whisk in the sugar. Zest about 2/3 of a lemon into the bowl. Juice the lemon into a measuring cup through a sieve. You will need 1/4 cup of fresh lemon juice. If your lemon is small, you may need 2 lemons, but one normally does the trick. Add the juice to the bowl. Whisk it together and then whisk in the flour.
Beat this until it is smooth and frothy. Pour the filling over the top of the hot crust.
Pop it back in the oven for 20-25 more minutes or until the top is just barley golden. Dust the top liberally with powdered sugar. Allow this to cool to room temperature and then chill it for an hour in the fridge. Cut into bars with a sharp knife and enjoy!