In the bowl of a stand mixer, whisk together 4 3⁄4 cups (594 grams) flour, 3⁄4 cup(150 grams) granulated sugar, 1 teaspoon(5 grams) orange zest, yeast, and1 teaspoon (3 grams) kosher salt until combined.
In a small saucepan, heat milk and 1⁄2 cup (113 grams) butter over medium heat until butter is melted and an instant-read thermometer registers 120°F(49°C) to 130°F (54°C). Add warm milk mixture to flour mixture; using the paddle attachment, beat at low speed until combined. Add 2 eggs (100 grams)and vanilla; beat until a wet, shaggy dough forms.
Switch to the dough hook attachment. Beat at medium speed until dough is smooth and elastic, 12 to 15 minutes; add up to 1⁄4 cup (31 grams) flour, 1 tablespoon (8 grams) at a time, if dough is too sticky. Turn out dough onto a lightly floured surface, and knead 5 to 8 times; shape into a smooth round.
Lightly oil a large bowl. Place dough in bowl, turning to grease top. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, 45 minutes to 1 hour.
In a small bowl, whisk together poppy seeds, remaining 1⁄2 cup (63 grams) flour, and remaining 1⁄2 teaspoon (1.5 grams) salt.
Clean bowl of stand mixer and paddle attachment. Using the paddle attachment, beat grapefruit zest, remaining 1 cup(200 grams) granulated sugar, remaining 3⁄4 cup (170 grams) unsalted butter, and remaining 1 tablespoon (15 grams) orangez est at medium speed until combined, about 2 minutes. Beat in poppy seed mixture until just combined.
Line 2 rimmed baking sheets with parchment paper.
Punch down dough, and let stand for 10 minutes. Divide dough into 10 portions (about 120 grams each). On a clean surface, roll each portion into an 8x5-inch rectangle, one long side facing you. Spread about 3 tablespoons (45 grams) grapefruit zest mixture on each rectangle, leaving a 1⁄4-inch border along one long side. Starting with long side opposite border, roll up each rectangle into a log, pinching seam to seal. Place 1 log seam side down, and cut in half lengthwise. Pinch one end together, and twist halves together, cut side up. Pinch remaining end, and bring into a knot, tucking ends under each other. Repeat with remaining logs. Place about 2 inches apart on prepared pans. Cover and let rise in a warm, draft-free place (75°F/24°C) until puffy, about 45 minutes.
Preheat oven to 350°F (180°C).
In a small bowl, whisk together 1 tablespoon (15 grams) water and remaining 1 egg (50 grams). Lightly brush over each bun.
Bake until puffed and golden and an instant-read thermometer inserted in center registers 190°F (88°C), 30 to 33 minutes. Let cool on pans for 10 minutes. Remove from pans, and let cool completely on a wire rack.
In a small bowl, whisk together confectioners’ sugar and orange juice until smooth. Drizzle over cooled buns. Best served same day.