Prepare the Crust:

Cup of Yum
1. Grease a 9-inch round tart pan with a removable bottom with nonstick spray.
2. In a bowl of a standing mixer fitted with a paddle attachment or in a large mixing bowl with a rubber spatula (or wooden spoon), combine all the crust ingredients: the butter, all-purpose flour, white whole wheat flour, powdered sugar, salt, vanilla, water, and lemon zest (if using). Mix until a somewhat dry looking cookie-dough-like mixture forms; when pressed together with your fingers, it should hold together. Press evenly along the bottom and up the sides of the prepared tart pan. Trim off any excess dough that overhangs the pan. Prick the dough all over with the tines of a fork.
3. Freeze the crust for 15 minutes. Meanwhile, place a rack in the center of the oven and preheat to 350 degrees F. Bake for 18 to 20 minutes, until golden. Let cool completely.

Make the Lemon Curd:

1. To a medium non-reactive saucepan*, add the butter and melt over medium-low heat.
2. Mix in the sugar, lemon juice, lemon zest, and egg yolks. Whisk frequently, about 5 minutes, until the sugar has dissolved—this workout will be worth it. Make sure you are using a small-ish whisk so you can really get into the corners of the pan.
3. Whisk in the cornstarch slurry. Increase the heat to medium and cook, whisking every 20 seconds. The mixture will thicken and bubble. Continue cooking, whisking very frequently, until the mixture is noticeably thickened (it will look like lemon curd!), about 1 to 2 minutes more.
4. If you are a perfectionist, strain the mixture through a fine mesh sieve to ensure that it’s fully smooth (if you’re happy with the consistency, you can skip this step).
5. Pour the hot lemon curd mixture over the cooled tart crust and, with a knife of offset spatula, spread into an even layer.
6. Allow the lemon curd to cool in the pie crust at room temperature for 1 hour, then place in the refrigerator to set for 4 hours or (for best results) overnight. Serve chilled with your favorite garnishes.