Crust

1.  Preheat oven to 350F and line an 8X8 or 9X9 square baking pan with foil so that the foil overhangs on the side and you can easily remove the bars from the pan later. Then grease every corner of foil.
2. In a large bowl add your flour, salt, and granulated sugar. Whisk to combine. Then pour in your melted butter, use a spatula to mix together. Place shortbread crust into the prepared pan and press down evenly making sure the crust fills the whole bottom of the pan. Bake for 18-23 minutes, or until nice and golden brown.

Lemon Filling

1. In a large bowl add the granulated sugar, all-purpose flour, cream of tartar and salt. Whisk together then add your eggs and egg yolks. Finally, add your freshly zested lemon and lemon juice. Transfer the mixture to a saucepan over low- medium heat.  Cook over low-medium heat, whisking constantly, mix until the mixture has thickened and your instant-read thermometer registers at a degree of 160F. This should around 5 minutes or a little longer, do not rush the process. Once the temperature is reached turn the heat off and add the slices of butter.
2. Pour the mixture through a strainer into a clean bowl. Then pour on top of the hot crust, moving pan to ensure an even filling. Bake for an additional 9-12 minutes or until when you shake the pan only the very middle barely wiggles.
3. Place pan on a wire rack to cool for at least 45 minutes. Then wrap in plastic wrap and chill in the fridge for at least 1 hour up to 24. Before serving using a paper towel gently place the towel on top of bars to remove any condensation. Then sift powdered sugar on top of bars for finishing touch. Cut into four rows on each side to create 16 bars. Keep in fridge, best served within 2 days.