Preheat the oven to 350ºF.

Mix together all the ingredients for the shortbread crust until a thick dough forms. Pat the dough evenly into an 8″x8″ baking pan, pressing the dough up the sides about 1″.

Prick the bottom of the crust a few times with a fork to prevent the crust from bubbling up. Par bake the crust, which means to cook until part way done, and golden brown, about 20 minutes.

While the crust is baking whisk together all the ingredients for the filling. Then cook the filling in a double boiler until a thermometer reads 170ºF, and the filling starts to thicken. To create a double boiler choose a large pot and fill part way with water.

Bring this to a boil, then maintain a simmer. Then whisk together the filling in a smaller pot placed inside of the bigger pot of simmering water. This creates a double boiler, and it’s only necessary to make sure the filling cooks evenly without curdling the eggs.

Once the filling is up to temperature, remove it from the heat source and pour the filling through a fine mesh sieve to remove any bits of egg and lemon zest. Then pour the hot filling into the hot crust, and bake until the filling is just set, but still jiggly in the middle, about 10 minutes.

Cool the lemon bars completely before slicing. For the best cleanest looking slices, refrigerate the lemon bars for 2-3 hours before serving. Refrigerate any leftovers for 2-3 days.