Oftentimes, lemon juice on its own will get lost in a cookie once it’s baked, but the flavor of the zest will shine through. Another place you can punch up the lemon flavor in these shortbread cookies is in the sugar that can roll them in before baking. I like to rub the zest of a lemon in the sugar so that you end up with a lemon sugar. Again, totally optional, but definitely adds another pop of lemon to these slice and bake cookies. Another option would be to use lemon extract, but it can sometimes be overpowering or taste "fake" instead of fresh.