Preheat oven to 350 degrees F. Grease a 9x13 inch baking pan with cooking spray, then line with a parchment paper sling (this makes removing and cutting the bars easier).

In a medium sized bowl, whisk together melted butter, sugar, and vanilla.

Whisk in flour and salt.

Dump into the prepared pan and use your hands to firmly press the mixture into an even layer.

Bake for 25 minutes, or until the edges are lightly browned.

Set aside to cool for 10 minutes while you make the filling.

Into a large bowl, sift in the granulated sugar, flour, cream of tartar, and salt. Set aside.

In a medium sized bowl, whisk together the eggs, lemon juice, zest, and vanilla extract.

Combine the two mixtures and whisk until well combined.

Pour over the warm crust and bake for 35 minutes, or until the center is relatively set and only has a slight jiggle.

Remove the bars from the pan, dust with powdered sugar (if desired), cut into squares, and enjoy cold!

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