Shortbread:
Place rack in middle of oven; preheat to 350°. Butter an 8x8" or 9x13" baking pan and line with a sheet of parchment paper, leaving at least 4" overhang on each side.
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Pulse 1 cup all-purpose flour, ¾ cup powdered sugar, ¾ cup granulated sugar, and ¾ tsp. kosher salt in a food processor a few times to combine. Cut  ½ cup (1 stick) into 1 inch pieces and add, pulsing until largest pieces are pea-size. Add 2 egg yolks and pulse just until dough comes together in small clumps. (Shortbread can also be made by hand: Whisk dry ingredients in a large bowl, then work in the butter with your fingers before mixing in the egg yolks with a sturdy spoon.)
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Transfer dough to the prepared pan (you may find it easier to form the dough into a ball first, instead of working with clumps) and press into an even layer. If the dough is sticky, use a piece of plastic wrap between your hand and the dough. Prick dough with a fork in several spots. Bake shortbread until edges and top are golden brown, 24–28 minutes. Let cool.
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Candied Grapefruit Zest:
While the shortbread is baking, use a vegetable peeler or sharp paring knife to remove zest from 1 grapefruit in 2"–3" wide strips (it’s okay to leave some white pith attached). Slice strips very thinly lengthwise; save fruit to juice for grapefruit curd or to eat.
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Combine zest and 2 cups water in a small saucepan and bring to a simmer. Drain immediately. Combine cooked zest with ¼ cup granulated sugar and 1 cup water in the same saucepan and bring to a simmer once again. Cook, stirring occasionally, until zest is translucent and softened, 15–20 minutes. Drain, discarding syrup, and toss zest in a small bowl with remaining ¼ cup granulated sugar. Set aside until ready to use.
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Curd and Assembly:
Reduce oven temperature to 300°. Vigorously whisk 3 large eggs, 3 egg yolks, 1 cup granulated sugar, and 1 Tbsp. cornstarch in a large saucepan until smooth and sugar is mostly dissolved.
Whisk in zest of 2 grapefruits and 3-4 lemons and a cup of juice total. (1/2 cup grapefruit juice, 1/2 cup lemon juice)
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Set saucepan over medium heat and cook, whisking constantly and making sure to get into the corners of the pan, until curd begins to bubble slightly around the edges and thickens, about 5 minutes.
Remove from heat and whisk ½ tsp. salt and 1 tsp. vanilla extract into curd, followed by butter a piece at a time, whisking to fully incorporate after each addition.
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Scrape curd into pan with shortbread and smooth over crust. Top with some of the candied grapefruit zest.
Bake grapefruit bars until surface of curd is set (the center will still wobble slightly when pan is gently shaken), 20–25 minutes. Let cool at room temperature for 30 minutes then transfer to the refrigerator and chill until very cold, at least 3 hours, or preferably overnight. Just before serving, slice grapefruit bars into 2" squares and dust with powdered sugar if desired.

Recipe adapted from Chris Morocco's “Grapefruit Bars with Candied Zest”