Crust: Line an 8x8 baking dish with parchment paper, or butter it.
Mix the flour and butter by mashing it with a wooden spoon until thoroughly combined. Then add the rest of the crust ingredients and keep mixing until it resembles a slightly crumbly cookie dough. Pat it into the baking dish (moisten your fingers with water to keep it from sticking to your hands.) Poke the crust all over with a fork.
Bake the crust at 350*F until golden, about 22 minutes.
Custard: Beat the eggs and yolk. Whisk in flour and sugar until smooth. Then add the lemon juice and zest and whisk for 2 minutes.
Bake at 350*F until the top looks a little white and crusty, which is about 25 minutes.
Let the bars cool completely before cutting them.