Prepare a square (8x8 or 9x9) glass baking dish with parchment paper with overhang on two sides (for easy lifting). Spray with nonstick cooking spray and set aside.
In a bowl, combine butter, flour, powdered sugar, and vanilla extract 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.
Using the parchment paper overhang, lift bars out of baking dish and place on cutting board or counter.
Slice into 16 even squares and then dust with powdered sugar and serve.
Video
Notes
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.