6tablespoonscold, unsalted butter, in 1/2" dice(3 ounces)
6tablespoonscold, unsalted butter, in 1/2" dice
1teaspoonfinely grated meyer lemon zest (from about 1 lemon)
3/4cupstrained meyer lemon juice (from about 5 lemons)
powdered sugar, for dusting
Make the crust:
Position a rack in the lower third of the oven and preheat to 350º.Line an 8" square pan with parchment paper on the bottom and up the sides. (Alternately, you can use thick aluminum foil, or an un-lined pan greased with butter.)
Combine the flours, sugar and salt in the bowl of a food processor. Add the butter cubes and pulse until no large butter chunks remain and the mixture begins to clump together into the size of peas. (You can also do this in a large bowl with your fingertips.) Dump the crumbs into the prepared pan and press the dough as evenly as possible into the bottom.
Bake the crust until it is golden all over, about 15-20 minutes.
Make the curd:
Set a mesh sieve over a heatproof bowl or large measuring cup and set aside. Place the diced butter and lemon zest in a small bowl and set aside.
In a medium, heavy-bottomed saucepan, whisk together the sugar, eggs and egg yolk to combine. Whisk in the lemon juice. Place the pot over medium-low heat, and cook, stirring constantly with a heatproof rubber spatula, until the mixture thickens slightly and reaches 160-170º on an instant-read thermometer, 5 - 10 minutes. As you stir, be sure to scrape the entire bottom and corners of the pan, so that the mixture heats as evenly as possible.
Immediately strain the mixture through the sieve to stop the cooking. Whisk in the butter pieces and zest until combined. (If you accidentally cook your curd too far and it begins to curdle, and stays curdled after you strain it and add the butter, you can probably rescue it by whizzing it in a blender or with an immersion blender.)
Bake the bars:
Pour the cooked curd over the baked crust. Bake the bars for about 20 minutes until the sides are barely puffed and the center of the bar wobbles like firm jell-o when you give it a gentle shake; it should not be wet or watery-looking (underbakenor should it be puffed in the center or cracking (overbaked).
Remove the bars from the oven and let cool for about 20 minutes, then chill until firm, 2-3 hours. Grasp the parchment and lift the bars from the pan and onto a cutting board; peel away the parchment. Trim away the outer edges, then use a large chef's knife to cut into 16 bars, wiping the blade clean between cuts. Just before serving, dust the tops with a bit of powdered sugar.
The bars keep well, refrigerated, for up to 3 days.