Measure ¾ cup (180ml) of warm water between 105°F and 110°F (40°C to 43°C). Pour into stand mixer bowl, add 2 tablespoons sugar and 2¼ teaspoons yeast. Stir briefly and let sit for 7-10 minutes until foamy.
Once yeast is foamy, add ¼ cup room-temperature whole milk, 1 large egg, and 3 tablespoons softened unsalted butter cut into pieces. Stir briefly to combine.
Whisk together 3 cups all-purpose flour and 1½ teaspoons fine salt. With mixer on lowest speed, gradually add flour mixture about ½ cup at a time until all is incorporated and dough comes together.
Increase mixer speed to medium and knead for 6-8 minutes until smooth, slightly elastic, and just barely tacky. Add flour 1 tablespoon at a time if dough is excessively sticky (maximum 2-3 extra tablespoons).
Lightly grease a large bowl with oil. Transfer dough to bowl, turn to coat all sides. Cover tightly with plastic wrap and let rise in a warm, draft-free spot until doubled in size, about 1 to 1½ hours.
Punch down dough and turn out onto lightly floured surface. Divide into 8 equal pieces (about 90g-100g each) using a kitchen scale. Shape each piece by folding edges inward and rolling under tension to create smooth, round balls. Flatten slightly to about 3 inches in diameter.
Line a baking sheet with parchment paper. Place shaped buns on parchment, spacing them at least 2 to 2.5 inches apart. Cover loosely with plastic wrap and let rise again for 45 minutes to 1 hour until puffy and soft.
Preheat oven to 375°F (190°C). Whisk together 1 large egg and 1 tablespoon water for egg wash. Gently brush tops and slightly down sides of each risen bun with egg wash. Sprinkle with sesame seeds if using.
Bake on center rack for 15-18 minutes until deeply golden brown. If browning too quickly, tent loosely with foil. Internal temperature should reach 190°F to 200°F (88°C to 93°C).
Transfer buns to a wire cooling rack immediately. Cool for at least 20-30 minutes before slicing to prevent gummy texture. Slice horizontally with a serrated bread knife and toast cut sides before serving.