Reduce oven to 325F. Set rack to middle position of oven.
Using a hand mixer or a stand mixer with the paddle attachment, beat the cream cheese, scraping down sides of bowl with spatula, until no lumps remain.
Gradually add the sugar, beating well after each addition.
Add the eggs, one at at time, beating well after each addition.
Add the sour cream, vanilla, lemon juice and zest, if using, and beat in.
Batter will be 'soupy' in texture. To ensure that no lumps remain, use a slotted spoon to check for lumps and incorporate into batter.
Pour batter over cooled base. Bake for 30-35 minutes, turning half-way through.
Bars are done when edges have slightly come away from the sides of the pan and appears set on top. (It will be somewhat jiggly in the center but will firm up with cooling.)
Set pan on rack and allow to cool 2-3 hours. If using a tinfoil sling remove from pan and cut bars to desired size. Bars can be served at this point or can be chilled for 3 to 24 hours.