We commonly make a double batch of it, using the zest and juice of five lemons, or two and a half per pan of bars. I always bake with large eggs, which works well for this custard. Rosie helped me make two pans of it last week to take to a celebration of Brian's birthday, and some folks asked for the recipe. Since I was typing it up anyway, I thought I would share it more widely.
1 cup butter or margerine, at room temperature
1/2 cup powdered sugar
2 cups flour
a pinch of salt
2 cups sugar
4 Tbsp flour
Juice and zest (the grated rind) of 2 or more lemons
(base recipe calls for 4 Tbsp of lemon juice)
Preheat oven to 350 degrees.
Combine crust ingredients in a mixer. Pastry will be crumbly.
Pack into 9x13 greased pan or baking dish(es). - I tend to spoon it into the pans then use my fingers to flatten it out and press it down.
Bake 15-20 min at 350 (will not brown).
Beat the eggs slightly, then blend custard ingredients in mixer.
On removal from oven, use wet knife to loosen pastry around edge. Pour custard over hot crust.
Bake it all 25-30 min at 350.
Sprinkle with powdered sugar while still warm.