For the Shortbread Crust

1. Preheat oven to 350 degrees. Make a parchment sling with two pieces of 9-inch wide by 13-inch long parchment, into a non-stick sprayed 9×9 inch square baking dish. Use clips if needed to secure the ends of the parchment, so the parchment doesn’t move when you are pressing the shortbread into the pan later.
2. In a bowl, whisk together the flour, sugar, and salt. Stir in the melted butter until a dough forms. Press the dough into the parchment lined baking pan evenly.
3. Bake for 20-22 minutes, rotating halfway through to ensure it is baked evenly. Remove when it is golden brown and place on wire rack until ready to fill with lemon curd. Keep the oven on. 

For Lemon Curd

1. Using the same bowl from the shortbread crust – no need to clean the bowl – whisk together the sugar, flour, cream of tartar, and salt.
2. Then whisk in the eggs and egg yolks, whisking until no white streaks from the eggs or sugar and flour remain. Continue to whisk in the lemon zest and the lemon juice.
3. Pour the mixture into a round bottomed sauce pan, and turn the stove on to medium heat. Stirring constantly, heat the mixture until it registers 160 degrees and thickens. This will ensure that the eggs are cooked through.
4. Turn off the heat, and add in the butter, one tablespoon at a time, whisking constantly until each one melts before adding the other.
5. Pour the lemon curd on top of the still warm shortbread crust. Spread the curd evenly if needed with the back of a spoon.

Bake The Bars

1. Bake the bars for another 10-12 minutes, or until they jiggle just slightly when shaken.
2. Remove them from the oven and allow to cool on a wire rack for 1 1/2 – 2 hours. Then refrigerate to cool completely before slicing.
3. Dust with powdered sugar if desired. Serve.