Shokupan
3 cups (375 g) bread flour, divided
1/3 cup plus 3 Tbsp. whole milk
2 large eggs
¼ cup (50 g) granulated sugar
3 Tbsp. heavy cream
2¼ tsp. active dry yeast
1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt
3 Tbsp. unsalted butter, cut into ½" pieces, room temperature
Nonstick vegetable oil spray or vegetable oil (for bowl)
Topping and assembly
1/3 cup (50 g) black sesame seeds
1 cup (110 g) powdered sugar
6 Tbsp. unsalted butter, room temperature
1 large egg
1 tsp. vanilla extract
2 cups (230 g) cake flour, sifted
½ tsp. baking powder
¼ tsp. kosher salt
All-purpose flour (for surface)
½ cup (100 g) granulated sugar
Shokupan
Whisk together ¼ cup bread flour (31 g), 1/3 cup whole milk, and ? cup cold water in a cold small saucepan. Place over medium heat and cook, whisking often, until a thick paste forms, about 2 minutes. Measure out ½ cup (125 g) tangzhong and discard any remaining. Let cool until lukewarm, about 15 minutes.
Combine tangzhong, 2 large eggs, ¼ cup (50 g) granulated sugar, 3 Tbsp. heavy cream, 2¼ tsp. active dry yeast, 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt, remaining 2¾ cups (344 g) bread flour, and remaining 3 Tbsp. whole milk in the bowl of a stand mixer fitted with the dough hook. Mix on low speed 3 minutes, then increase speed to medium and continue to mix, scraping down sides of bowl occasionally if needed, until dough is smooth, about 10 minutes.
With motor running, gradually add 3 Tbsp. unsalted butter, cut into ½" pieces, room temperature, a couple pieces at a time. Once butter is fully incorporated, continue to mix 3 minutes (dough will be smooth and slightly tacky).
Lightly coat a large bowl with nonstick vegetable oil spray or vegetable oil and transfer dough to bowl; turn dough to coat. Cover with a lid, large plate, or plastic wrap and let sit in a warm, draft-free spot until doubled in size, 1½–2 hours.
Topping and assembly
Meanwhile, finely grind black sesame seeds in spice mill (stop grinding before seeds turn to a paste).
Beat 1 cup (110 g) powdered sugar and 6 Tbsp. unsalted butter, room temperature, in the clean bowl of a stand mixer fitted with the paddle attachment on medium speed until smooth and creamy; scrape down sides of bowl with a rubber spatula. Add 1 large egg and 1 tsp. vanilla extract and beat until well combined, about 2 minutes (mixture will look slightly curdled; that’s okay). Add ground sesame seeds, 2 cups (230 g) cake flour, sifted, ½ tsp. baking powder, and ¼ tsp. kosher salt and mix on low speed, scraping down sides occasionally, until a dough forms, about 2 minutes.
Divide black sesame dough in half. Working with one half at a time, roll out dough between large sheets of parchment paper to 1/8" thick. Stack sheets of dough, still sandwiched between parchment, then slide onto a baking sheet and chill until firm, at least 45 minutes, or until ready to use.
When shokupan is doubled in size, punch down and turn out onto a lightly floured surface. Divide dough into 14 equal pieces (50 g each if using a scale). Working one at a time, flatten each piece with your palm, then fold all edges up toward the center and pinch together to close (it should look a little like a dumpling). Turn dough seam side down. With your hand cupped over dough (dough should be positioned between the edge of your hand and the fleshy part below your thumb), roll dough in a circular motion, keeping seam constantly touching surface, until a tight ball forms. Turn seam side up.
Working with 1 sheet at a time, remove black sesame dough from fridge. Peel off top sheet of parchment and lightly place it back over dough; turn dough over and remove second sheet of parchment. Punch out rounds with cutter. Using a small offset spatula, lift rounds off parchment and drape over shokupan balls (ensure black sesame dough covers seam on shokupan). Gently mold black sesame dough around shokupan by cupping in your palm (cookie will not cover the entire ball). If rounds become too soft to work with at any point, chill until firm but pliable before continuing, 5–10 minutes.
Place ½ cup (100 g) granulated sugar in a small bowl. Working one at a time, roll cookie topping side of each bun in sugar. Divide buns between 2 parchment-lined baking sheets, spacing about 2" apart. Using a sharp paring knife, carefully score top of each bun to make a grid, being careful not to slice into shokupan. Cover buns loosely with plastic wrap and let sit in a warm, draft-free spot until doubled in size, about 1 hour.
Place racks in upper and lower thirds of oven; preheat to 350°F. Bake buns, rotating bakingsheets top to bottom and front to back halfway through, until bread is golden brown (an instant read thermometer will register 200°F), 17–20 minutes. Cookie topping will spread and crackle as buns bake. Let buns cool on baking sheets until warm before serving, about 15 minutes.