Preheat oven to 350 F. In a large bowl using either a hand or stand mixer, beat softened butter with sugar until fluffy.
Add flour and salt and beat until a thick dough forms. Press cookie dough into a baking dish. Bake for 15-20 minutes.
In a large bowl, whisk together sugar, eggs, lemon zest, lemon juice and flour. Pour lemon custard over the baked shortbread crust. Bake for another 20-25 minutes, until the lemon filling is set.