1tspcoarse Kosher salt(if using table salt, use half the amount)
¾cupcold unsalted butter,diced
Crust and Topping
Preheat the oven to 350 F. Line an 8x8 inch baking dish with a piece of parchment paper.
In a bowl, combine the flour, sugar and salt. Scatter the cold pieces of butter over the flour mixture, and use a pastry cutter to cut the butter into the flour until moistened, crumbly, and no visible pieces of butter remain.
Measure out 1 cup of the crumb mixture, and set aside to reserve for the crumb topping.
Press the remaining crumbs firmly against the bottom of the pan. Bake for 15 minutes.
In a bowl, whisk together the eggs, sugar and flour until smooth.
Use a microplane zester to zest 3 of the lemons, and whisk the lemon zest into the eggs and sugar mixture.
Juice the lemons, measuring 1/2 cup juice. If needed, juice the 4th lemon to get a full 1/2 cup of lemon juice. Whisk the lemon juice into the egg mixture until smooth.
After the crust has baked for 15 minutes, remove from the oven, and cool for 5 minutes.
Whisk up the filling again, and carefully pour over the hot crust. Bake for 15 minutes.
Gently sprinkle the reserved 1 cup of crumbs over the filling, then continue baking for another 20 minutes, until the topping is a pale golden.
Let cool at room temperature for at least 2 hours, before slicing. These bars can be served slightly warm, or at room temperature.
Leftover bars should be stored in an airtight container, and are best eaten within 1-2 days.