Preheat oven to 350 degrees. Prepare a square (8x8 or 9x9) glass baking dish with parchment paper and nonstick cooking spray, set aside.
In a bowl, combine butter, flour, powdered sugar, and vanilla together until a dough forms. Press dough into the bottom of the pan, evenly. Bake 12-15 minutes or until the edges are just barely golden.
While the shortbread bakes, make the lemon custard by whisking sugar, lemon juice, eggs, and flour together.
As soon as the crust is done baking, cover with lemon filling and return to the oven for 20 minutes or until set. Cool lemon bars completely.
Cut into 16 squares and then dust with powdered sugar.
If the dough isn't sticking together your butter is probably too cold. To fix this, mix with clean hands to warm the dough up.