Add water, starter, egg yolks, sugar, salt, flour, and soft butter to a stand mixer. Mix on low for 2–3 minutes until combined. Rest 10–30 minutes. Then mix on medium for 6–10 minutes until the dough is smooth, elastic, and stretches thin. (If kneading by hand, knead 12–15 minutes with a brief rest or two.)

Transfer dough to a clean bowl.

Let it bulk ferment at room temperature for 6–8 hours until puffy and jiggly with about a 50–70% increase. You can do 1–2 gentle stretch-and-folds in the first hour if you like. If your kitchen is warm, move the bowl to a cooler spot to prevent overproofing.

Lightly flour your work surface. Divide the dough into about 15 pieces (~40 g each). Shape each piece into a tight ball by tucking the edges underneath and rolling against the counter. If the dough feels hard to handle, cover and chill for 15–20 minutes before finishing shaping.

Place the shaped balls in a buttered 9×13-inch baking dish.

Cover loosely and let them proof at room temperature for 1–2 hours until very puffy. In colder kitchens, you may need 4–6 hours.

Preheat oven to 450°F (232°C). Brush rolls with egg wash and sprinkle with sesame seeds. Bake at 450°F (232°C) for 10 minutes. Reduce temperature to 375°F (190°C) and bake for another 10 minutes, checking a center roll for an internal temperature of ~205°F (96°C).

Cool the rolls for 10 minutes in the pan. Transfer to a rack and cover with a towel. Serve within 24 hours or freeze for up to 2 months.