Doughnuts with Grapefruit Curd and Citrus Sugar

Ingredients:
1 tablespoon finely grated grapefruit zest
1 cup fresh white grapefruit juice, strained
4 large egg yolks
2 large eggs
1/2 cup sugar
1/2 teaspoon kosher salt
1 teaspoon finely grated lemon zest
2 tablespoons fresh lemon juice
4 tablespoons unsalted butter, cut into pieces
1/3 cup sugar
1 tablespoon finely grated grapefruit zest
3/4 cup whole milk
1/2 vanilla bean, split lengthwise
3 tablespoons sugar, divided
1 (1/4-ounce) envelope active dry yeast (about 2 1/4 teaspoons)
1 large egg
1 large egg yolk
2 tablespoons honey
1 teaspoon finely grated lemon zest
1 teaspoon kosher salt
2 3/4 cups all-purpose flour, plus more for dusting
4 tablespoons unsalted butter, room temperature, cut into pieces
Vegetable oil (for frying; about 6 cups), plus more for bowl
A 2 1/4–2 1/2-inch round cutter; a deep-fry thermometer

Instructions:
Bring grapefruit juice to a simmer in a small saucepan over medium-high heat and cook until reduced to 1/3 cup, 12–14 minutes. Let cool slightly.
Whisk egg yolks, eggs, sugar, and salt in a medium heatproof bowl to combine. Whisk in grapefruit zest, lemon zest, lemon juice, and reduced grapefruit juice. Set bowl over a large saucepan of simmering water. Cook, whisking almost constantly, until curd is thickened, 6–8 minutes. Remove from heat and whisk in butter, adding a few pieces at a time.
Transfer to a nonreactive bowl and press a piece of plastic wrap directly onto surface of curd. Chill until set up and very cold, at least 4 hours and up to 12 hours.
Combine sugar and zest in a small bowl. Let sit uncovered 12 hours to dry out. (You can also microwave on high in 15-second intervals, stirring and letting cool slightly between intervals, until dehydrated.)
Transfer to a blender and blend until powdery. (Or, work zest into sugar with your fingers; just make sure to break up any clumps.)
Heat milk in a small saucepan or microwave until just warm (you want it to be between 105°F and 110°F). Transfer to the bowl of a stand mixer and scrape in vanilla seeds; reserve pod for another use. Add 1 Tbsp. sugar and sprinkle yeast over. Let sit until yeast starts to foam, about 5 minutes.
Add egg, egg yolk, honey, lemon zest, salt, and remaining 2 Tbsp. sugar. Mix gently with a wooden spoon until barely combined. Add 2 3/4 cups flour and knead with a dough hook on medium-high speed until dough comes together into a mass that no longer sticks to the sides of bowl and is beginning to climb up the hook, about 5 minutes. (Dough should spring back immediately when pressed.) Reduce speed to medium-low; add butter a couple of pieces at a time, working in completely after each addition.
Turn out dough onto a work surface and knead with your hands until smooth and springy, about 2 minutes. Transfer to a very lightly oiled bowl, cover with plastic wrap, and let rise in a warm spot until doubled in volume, about 2 hours.
Turn out dough onto a lightly floured surface and roll out 1/2" thick. Cut out doughnuts with cutter, twisting cutter to release them. Reroll any scraps. Transfer to a very lightly floured parchment paper–lined baking sheet. Cover with a kitchen towel; let sit in a warm spot until slightly puffed and dough springs back lightly when pressed, 40–45 minutes.
Fit a wide heavy pot with thermometer; pour in oil to come at least 2" up sides. Heat over medium-high until thermometer registers 325°F. Fry 3 or 4 doughnuts at a time, turning halfway through, until deep golden brown, about 6 minutes per batch. Transfer to a wire rack set inside a rimmed baking sheet; let cool.
Using a paring knife, poke a hole in the top of each doughnut, being careful not to push through to the opposite side. Transfer grapefruit curd to a piping bag or resealable plastic bag (snip off one corner) fitted with a 1/4” round tip. Fill doughnuts with grapefruit curd, then coat generously with citrus sugar.
Citrus sugar can be made 3 days ahead. Store airtight at room temperature.