Prepare the base:
Preheat the oven to 180°F (350°C).
In a large bowl, mix the flour, powdered sugar, and salt.
Add the cold cubed butter and work with your fingers or a hand blender until the mixture is sandy.
Press the mixture evenly into the bottom of a square baking pan of about 23×23 cm, lined with baking paper.
Bake in a preheated oven for 15-20 minutes, or until the base is lightly browned. Remove from the oven and allow to cool slightly.
Prepare the lemon filling:
In a bowl, beat together the eggs and sugar until smooth.
Add the flour and cornstarch, stirring well to avoid lumps.
Add the lemon juice and grated zest, stirring until everything is well combined.
Assemble and bake:
Pour the lemon filling over the cooked base and even out the surface.
Return the baking dish to the oven and bake for an additional 20-25 minutes, or until the filling is set and lightly browned on the edges.
Remove from the oven and allow to cool completely in the pan.
Decorate and serve:
Once cooled, transfer the lemon squares to a cutting board and cut them into evenly sized squares.
Sprinkle with powdered sugar before serving.
