For dough: In the bowl of a stand mixer fitted with the paddle attachment, beat 1½ cups (191 grams) Bread Flour, Granulated Sugar, Active Dry Yeast, Kosher Salt, and Freshly Ground Cardamom Seeds at low speed just until combined.
In a medium saucepan, heat Whole Milk and Unsalted Butter over medium heat, stirring occasionally, until the 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; beat at medium speed until combined, 1 to 2 minutes. Add Egg; beat at medium-high speed for 2 minutes. Gradually add remaining 2¼ cups (286 grams) Bread Flour, beating just until combined and stopping to scrape bottom and sides of the bowl.
Switch to the dough hook attachment. Beat at medium-low speed until a soft, smooth, elastic dough forms, 6 to 10 minutes. (Dough may still stick slightly to sides of the bowl but should pass the windowpane test.) Turn out dough onto a clean surface, and shape into a round.
Oil a large bowl. Place dough in bowl, turning to grease the top. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, 45 minutes to 1 hour.
Punch down dough. On a clean surface, pat dough into a 10x6-inch rectangle. Wrap in plastic wrap, and refrigerate for at least 8 hours or up to overnight.
For cinnamon filling: In a medium bowl, stir together Unsalted Butter, Light Brown Sugar, Bread Flour, Ground Cinnamon, and Kosher Salt until well combined.
Alternatively, for cardamom filling: In a small bowl, whisk together Granulated Sugar and Freshly Ground Cardamom Seeds; reserve ¼ cup (50 grams) cardamom sugar in another small bowl. Stir Butter, Bread Flour, and Kosher Salt into remaining cardamom sugar until well combined.
Let dough stand at room temperature for 5 minutes. On a lightly floured surface, roll dough into a 25x10-inch rectangle, with one long side closest to you. Using an offset spatula, dollop desired filling onto dough, and spread in a thin, even layer to edges. Working from the short sides, fold dough in thirds like a letter.
Line 2 rimmed baking sheets with parchment paper. Roll dough into an 11x8-inch rectangle, with one long side closest to you. Using a pastry wheel or sharp knife, cut dough lengthwise into 15 strips (about ¾ inch wide each). Holding up one end of 1 strip vertically, gently pull dough in increments until the strip is 22 inches long, being careful to keep an even thickness throughout dough so it does not tear. Holding the bottom end of the strip between index and middle fingers and thumb of one hand, loosely wrap dough strip three times around fingers of that same hand, overlapping dough slightly. Place the thumb of the same hand over overlapped dough strands to secure; wrap the remaining end of dough perpendicularly, between index and middle fingers, and around overlapping dough strands to form a knot-like shape, tucking the loose end under the bottom of the finished bun as you remove your fingers. Repeat with remaining strips, and place about 2 inches apart on prepared pans.
Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size and dough holds an indentation when gently pressed, 1 to 1½ hours.
Preheat oven to 350°F (180°C). For topping: In another small bowl, whisk together Egg and 1 tablespoon (Water); brush onto buns. Sprinkle with pearl sugar (if using) or reserved cardamom sugar as desired.
Bake until golden brown and an instant-read thermometer inserted in the center registers at least 190°F (88°C), 15 to 20 minutes, rotating pans halfway through baking. Immediately sprinkle with additional pearl sugar (if using) or any remaining reserved cardamom sugar as desired. Let cool on pans for 5 minutes. Serve warm.
