Crust

1.  Preheat oven to 350F/180C and line an 8X8 or 9X9 square baking pan with parchment paper and spray with non-stick baking spray.
2. In a large bowl, add your flour, salt, and granulated sugar. Whisk to combine. Then pour in your melted butter, and use a spatula to mix together. Place the shortbread crust into the prepared pan and press down evenly, making sure the crust fills the whole bottom of the pan. Bake for 15-20 minutes, or until nice and golden brown. (Closer to 15 minutes for a 9 inch pan and 20 minutes for an 8 inch pan)

Raspberry Swirl & Lemon Filling

1. In a blender or food processor, mix together the raspberries and sugar until completely blended. Place a fine mesh sieve over a clean bowl and pass the raspberry mixture through to remove the seeds. Set aside.
2. 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 temperature of 160F. This should take 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.
3. Pour the mixture through a strainer into a clean bowl. Then pour on top of the hot crust, moving the pan to ensure an even filling. Dollop the raspberry sauce on top, then use a butter knife to swirl the design. Bake for an additional 9-12 minutes or when you shake the pan only the very middle barely wiggles.
4. Place the 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 a finishing touch. Cut into four rows on each side to create 16 bars. Keep in an airtight container in the fridge, best served within 2 days.