Activate the Yeast: In a small bowl, combine milk and water heated to 110°F. Add about 2 teaspoons of the sugar. Stir to dissolve, then sprinkle the yeast over the top. Let sit for 5-8 minutes until foamy and active. *If the mixture doesn't foam, your yeast may be old or the liquid temperature incorrect; start over with fresh yeast.
Mix Dry Ingredients: In the bowl of a stand mixer, sift together the bread flour, buttermilk powder, remaining sugar, and salt. Use a spatula to work through any remaining clumps in the sifter until the mixture is smooth and uniform.
Form the Dough: Attach the dough hook to your mixer. Turn to low speed and pour in the activated yeast mixture. Mix until the dough begins to come together, about 2 minutes. With the mixer running on low, gradually add the softened butter a little at a time. Once all butter is incorporated, increase speed to medium and knead for 5-8 minutes until the dough pulls cleanly away from the sides of the bowl. The dough should be slightly tacky but not sticky, and should clean off your hands easily.
Adjust Consistency if Needed: If your dough is too wet and not forming a ball, add flour 1 tablespoon at a time, mixing well after each addition, until it reaches the proper consistency. Different flour brands and humidity levels affect hydration.
First Rise: Turn the dough out onto a clean surface and shape into a smooth ball by pulling the edges toward the center and pinching to seal. Lightly oil the mixing bowl. Place the dough ball seam-side down back into the oiled bowl. Cover with a tea towel and let rise on the counter for 1 hour. Dough should be doubled in size.
Divide the Dough: Punch down the risen dough to release gas. Divide into 6 equal portions, approximately 5.9 ounces each if using a scale. If not using a scale, eyeball 6 equal pieces.
Prepare Baking Sheet: Line a baking sheet with parchment paper. Then dust with the cornmeal.
Shape the Rolls: Working with one piece at a time, gently roll and stretch the dough into an 8-inch log. Work gently to avoid pressing out too much air. Let the dough naturally form a cylinder as you roll it between your palms and the counter, smoothing out any large air pockets. Avoid overworking the dough.
Add Seeds: Sprinkle sesame seeds onto your work surface. Roll each shaped log in the sesame seeds to coat the sides and top. Place seeded rolls on the prepared baking sheet, spacing them about 2 inches apart to allow for expansion.
Second Rise: Cover the shaped rolls loosely with plastic wrap or a tea towel. Let rise on the counter for 45 minutes until puffy and expanded.
Preheat Oven: While the rolls rise, preheat your oven to 375°F.
Prepare Egg Wash: In a small bowl, whisk together the egg yolk, melted butter, and hot water until smooth.
Score and Glaze: Using a sharp knife, razor blade, or bread scoring tool, make 3-4 diagonal slashes across the top of each roll at a 45-degree angle, or one long slit along the length of the top. Brush the egg wash generously over each roll, making sure to coat the entire surface. Lightly sprinkle kosher salt over the tops.
Bake: Bake for 16-18 minutes, or until the rolls are golden.
Cool: Transfer rolls to a wire rack and let cool for at least 15-20 minutes before slicing. The bread will continue to set as it cools.