Preheat the oven to 350°F (175°C).
In a medium bowl, mix together softened butter, 2 cups of flour, and 1/2 cup sugar until well combined. Press the mixture into a 9x13 inch pan.
Bake the crust in preheated oven for 15 to 20 minutes, or until firm and golden.
In another bowl, whisk together the eggs, 1 1/2 cups sugar, 1/3 cup lemon juice, remaining flour, and lemon zest. Pour over the baked crust.
Bake for an additional 20 minutes in the preheated oven. The bars will firm up as they cool.
Allow to completely cool before cutting into squares and serving.