Lemon-Rhubarb Bars

 

2 sticks (1/2 pound) unsalted butter, softened, plus more for greasing pan
2 cups all-purpose flour
1/2 cup granulated sugar
1/2 tsp. kosher salt

3/4 pound (6–10 stalks, depending on size) red rhubarb, chopped
1/2 cup water
1/3 cup plus 2/3 cup granulated sugar
2 T. fresh lemon juice
1/2 tsp. freshly grated lemon zest
2 large eggs, plus 2 egg yolks
1/3 cup all-purpose flour
Powdered sugar, for dusting

 

Preheat your oven to 350° and set a rack to the middle position. Grease the bottom of a 9-by-13-inch baking pan, then line with a piece of parchment paper that’s long enough to drape over the sides of the pan. In a large bowl, use a handheld or stand mixer on low speed to combine the butter, flour, sugar, and salt until they come together as a dough. Press this evenly into the bottom of the baking pan, then bake until it’s just beginning to turn golden, 15 to 20 minutes. In a 3- or 4-quart pot, combine the chopped rhubarb with the water and 1/3 cup sugar. Set over medium-high heat, cover, and simmer, stirring occasionally, until the rhubarb is completely broken down, 8 to 10 minutes. Remove from heat and let sit for 5 minutes. Use an immersion or standing blender to completely purée the rhubarb, then add the remaining 2/3 cup sugar, lemon juice, and lemon zest. Stir and set aside. In a large bowl, whisk together the eggs and yolks. Once the rhubarb purée is cool enough to touch, add it to the eggs and whisk to combine, then stir in the flour until smooth. Pour the topping over the shortbread crust and bake until it’s set and barely beginning to turn golden at the edges, 20 to 25 minutes. Remove from the oven and let cool at room temperature for 30 minutes, then chill, uncovered, for at least 3 hours. Run a knife along the edge of the pan to loosen the slab, then lift it out in one piece and cut into equal squares. Just before serving, dust with powdered sugar.