Preheat the oven to 350°. Grease a 9x13 pan (or an 8x10 pan if you want the layers to be thicker).
In a bowl, stir together the flour, sugar, and salt.
Add the butter (cut into small cubes) to the flour mixture and use a pastry cutter (or 2 knives together) to cut it all together until the mixture resembles fine crumbs.
Press into the bottom of the prepared pan and bake until golden around the edges - about 20 minutes.
In a bowl, stir together the sugar and flour.
Add the eggs and whisk to combine.
Add the lemon zest and juice and mix until combined.
Pour over the crust and bake an additional 20 minutes.
Cool then chill in the fridge for at least 2 hours. Sift powder sugar over the top before cutting into squares.