Preheat your oven to 350 degrees. In a glass bowl, place the pats of butter and melt them either with the use of a microwave at 30 second increments, or with the use of a double broiler, over your stove top. The butter should be almost completely melted, with only some slight solids left.
Prepare your 8"x8" pan by dusting and spraying it. I use a piece of parchment paper at the bottom with two sides cut to come up and out so that it's easy to remove the bars and cut them after they are set.
Onto the almost totally melted butter, add vanilla extract, sugar and salt. Mix to combine. Add all of the flour. Mix until everything is combined. Use your hands to combine the dough into a ball and then kneed the dough down to be placed evenly to the bottom of the pan.
Use the bottom of your measuring cup to apply pressure and try to get the dough to be as smooth and even as you can. It should be one solid piece with no cracks. Use the tip of a fork to make slight impressions to the top of the dough, about three rows across.
Bake for 10-15 minutes, or until it has turned golden and according to what you'd like to enjoy. A softer color means a softer bite. A darker or more intense color, means the bite will be more crisp. For my oven at home, 13 minutes is the just-right time frame.
Once the shortbread is baked. Take it out to cool. While you are making the lemon curd, it will be ready to handle.
In a bowl, with a blending or beater attachment, place the softened butter and cream it to the bottom of the bowl. Add the sugar and salt. Mix to combine. Add the eggs. I add them two at a time. I always go with adding any separated eggs first, just in case I make a mistake on the separating. So first combine the two yolks, and then combine the two whole eggs. Add the corn starch. Add the fresh squeezed lemon juice last.
Pour this mixture into a saucepan. On medium high heat, start to whisk. Continue to whisk the mixture until it comes to 180 degrees. Use a hand-held thermometer to check the temperature throughout the whisking process. Once it reaches the 180 degrees temperature, remove from heat.
Pour this to the top of the cooled shortbread. Smooth out so that it's an even layer. Allow to chill in the refrigerator and set for about three hours.
Once you're ready to cut, remove with the paper and place onto a cutting board. Place the bars onto a plate and if you like the option, dust with a little powdered sugar. These are best kept in the fridge if you are not serving all at once.