• Prepare the tart pan: Grease a 9" tart pan with butter or non-stick cooking spray. Add some flour and toss to coat the sides and bottom, then shake out the excess. Greasing the pan and coating with flour makes it easier to get the crust out of the pan.

Make the Shortbread Crust:

• Melt the unsalted butter in a saucepan or melt in the microwave in 10-second intervals, stirring between each.
• In the bowl of a food processor, combine the flour, sugar, salt, egg yolks, and fresh-squeezed lemon juice. Pulse until combined. Add the melted butter and pulse until the dough comes together.
• Dump the dough into the prepared tart pan and use your hands or the flat side of a glass to pack it tightly and evenly into the bottom and around the sides of the pan. Use a fork to poke a few holes in the bottom of the dough.
• Place the tart pan in the freezer and begin preheating your oven to 350°.
• Once the oven is preheated, place the tart pan on a large-rimmed baking sheet and pre-bake the crust for 15 minutes.

Make the Lemon Curd Filling:

• In a large bowl, vigorously whisk the eggs, egg yolks, sugar, and salt until smooth and pale-yellow.
• In a separate bowl, whisk the flour, fresh-squeezed lemon juice, and vanilla extract until combined and the flour is dissolved.
• Melt the unsalted butter in a medium pot. Add the lemon juice/flour mixture and cook, whisking constantly, until starting to bubble and thicken (about 2 minutes). Remove from heat and slowly pour the hot liquid into the bowl with the eggs. Whisk the eggs vigorously as you do so they don't curdle.
• Once all of the hot liquid has been added to the eggs, whisk until the mixture is completely smooth, then pour everything back into the pot. Bring the curd to a boil, whisking constantly, until very thick (about 7-10 minutes). Note: Once the curd starts to boil it will thicken rapidly, if it's thickening too fast for you to whisk it and keep it smooth, lower the heat or take the pan off the heat for a minute and whisk until it smooths out.
• Once it's thickened to the desired consistency (think custard or pudding consistency), immediately strain the lemon curd through a sieve and into a bowl. Use a whisk to help get it through the sieve, then use a rubber spatula or the back of a spoon to press the rest of it through. Use a different spatula or spoon to scrape the strained curd from the other side and into the bowl.
• Pour the warm lemon curd into the pre-baked crust and use an offset or rubber spatula to spread it out smoothly and evenly. Bake the lemon tart for 15 minutes.
• Take the tart out of the oven and allow it to cool completely at room temperature, then in the fridge for at least 2 hours before slicing and serving.