Heat the oven to 350°F with a rack in the middle position. Line an 8"x8" baking dish with parchment, laying down in one direction length wise then one piece width wise. Leaving about 4 inches of extra parchment at all ends.

Beat together the butter and powdered sugar in a stand mixer (or with a hand mixer) on medium-high speed until they are completely combined, turn pale yellow, and resemble creamy frosting. Stop the mixer and scrape down the sides of the bowl as needed while mixing.

Add the flours, and salt to the bowl. Beat on low speed just until all the flour is incorporated and the mixture comes together into a dough. The dough will seem fairly crumbly, but should hold together when squished in your hand.

Press the dough into the baking dish in an even layer. Prick with a fork all over. Bake for 20 to 25 minutes, until the edges of the shortbread turn golden-brown.

Now for the lemon curd whisk together the eggs, yolks, and sugar in a small (2-quart) saucepan, whisk in the lemon zest, lemon juice, and salt to form a smooth, liquidy mixture.

Place the pan with the lemon mixture over medium heat. Stir, gently but continuously, until the mixture thickens enough to coat the back of a spoon and registers about 155°F on an instant-read thermometer. This should take 8 to 10 minutes. Be sure to scrape the bottom and edges of the pan as you stir. If you notice the mixture starting to coagulate and clump up, immediately remove the pan from heat. Stir in the butter until completely melted.

When the shortbread crust is ready, remove it from the oven and pour the curd over top. Bake at 350°F until the edges of the curd are set, but the middle is still jiggly, 10 to 15 minutes.

Cool completely on the counter, and then cover and refrigerate for at least 4 hours (or overnight). Remove from pan by grasping ends of parchment paper and set down on cutting board. Slice into squares and serve. Bars will keep refrigerated for several days.