In a small bowl, sprinkle the yeast over the warm water and let it stand for 5 minutes. Then, stir to dissolve the yeast.
Mix the cold water and warm milk together in the bowl of a heavy stand mixer (or large bowl) to cool the milk to tepid.
Add the dissolved yeast, melted butter, sugar, egg yolk, and salt.
On low speed, with the paddle attachment, mix enough of the flour to make a soft dough that cleans the sides of the bowl.
Change to the dough hook and mix on medium-low speed, adding more flour as needed, until the dough is smooth, elastic, and tacky - about 6 minutes.
Lightly grease a medium bowl. Shape the dough into a bowl and put the dough in the bowl, turn the ball to coat it, and leave smooth side up.
Cover the bowl with plastic wrap. Let stand in a warm, non-drafty, place until it is double in volume, about 1½ to 2 hours. (See NOTES about proofing)
Lightly butter a 7"x11" baking dish. Turn the dough out onto an un-floured work surface and knead briefly.
Divide the dough into 12 equal pieces and shape each into a taut ball.
Place the balls, smooth sides up, equally spaced, in the baking dish.
Cover loosely the dish with plastic wrap and let stand in a warm place until almost doubled in volume, about 30 minutes to an hour.
Position a rack in the center of the oven and preheat the oven to 350°F.
Lightly brush the tops of the balls with some of the milk and sprinkle with sesame seeds.
Bake until the rolls have fully risen and golden brown, 25 to 30 minutes.
Remove from the oven and let cool in the pan for 10 minutes. Use a knife to cut the rolls and remove them with a small spatula or fork.
Serve warm or cooled to room temperature.