Starting from the bottom up, these bars begin with a simple, buttery shortbread crust. Nothing funky, just 3 ingredients pulsed in the food processor to make a perfect base. Next, the combination of both lemon juice and lemon zest gives a bold citrus flavor. Note that fresh juice and zest are a must. None of the jarred lemon juice or off-the-rack dried lemon zest. Adding flour to the lemon mixture thickens the filling perfectly without compromising the texture. Having the perfect ratio of crust to filling is key, too. You need just enough crust so you can pick up these bars with your fingers as needed and a thicker layer of lemon custard to give you that zesty lemon flavor you love!