FOR THE SHORTBREAD
4 oz. (8T) unsalted butter, at room temperature
2 T. granulated sugar
1 T. confectioners sugar
1/2 t. vanilla
1/2 c. all-purpose flour
2/3 c. cake flour
1/4 t. baking powder
1/4 t. salt
FOR LEMON CURD
1 c. fresh lemon juice (4 to 6 lemons)
4 T. unsalted butter, cut into 2 pieces
2 T. heavy cream
1 c. granulated sugar
4 large eggs
2 large egg yolks
1/4 t. salt
1/4 t. vanilla
TO MAKE SHORTBREAD
In a large bowl, cream together butter and both sugars until light and fluffy (about 5 minutes) Beat in the vanilla until throughly combined, scraping down the sides of bowl.
In medium bowl, sift together both flours, baking powder and salt. Slowly blend the dry ingredients into the wet ingredients, scraping down the sides, until flour is completely blended and the dough is homogeneous.
Scrape the dough from the bowl onto a sheet of plastic wrap. Wrap well and press down to for a 1/2" square. Refrigerate the dough until it;s firm but still pliable, about 20 minutes. Heat the oven to 350º. Prepare two sheets of parchment or waxed paper, grease an 8x8 metal or glass baking pan with butter.
When the dough is firm, unwrap it and put it between the sheets of parchment or paper. Roll the dough to an approximate square, slightly larger than 8x8" baking pan and about 1/4" thick. Trim the dough to fit pan and press the dough into the bottom of the pan, leaving the parchment on the bottom. Bake until the shortbread is light golden on top, 25 to 30 minutes, Remove the pan from the oven but keep the heat set at 350º as you make the lemon curd.
TO MAKE THE LEMON CURD
In a medium saucepan, heat the lemon juice, butter and cream into just under a boil; the butter should be melted Remove from the heat.
In a medium bowl, whisk together by hand the sugar, eggs, and yolks until combined. Whisk in a bit of the hot liquid and then gradually whisk in a bit more until all is added.
Pour the mixture back into the saucepan ad heat on medium, stirring constantly with a wooden spoon, scraping the bottom and sides of the pan to keep the eggs from scrambling. Cook until the lemon curd coats the spoon thickly enough to leave a line when you draw your finger through, 5 to 8 minutes. Remove from heat and strain through a sieve. Stir in vanilla and salt.
Pour the curd over the baked shortbread and smooth it evenly with a spatula. Bake until curd has set and jiggles like jello, 15 to 20 minutes. Let cool to room temperature. Gently tug on the parchment on all sides to loosen the bars from the pan. Lift them out and onto a cutting board and refrigerate until the curd has set, at least 4 hours. Trim the sides for a cleaner look and cut into 16 pieces.