To make the dough, in a bowl, dissolve the yeast in the water.
Sift the flour into the bowl of a stand mixer fitted with the hook attachment. Start the mixer on a slow speed, and slowly add all the yeast mixture. Knead for a few minutes until you have a smooth and firm ball.
Now it's time to slowly add the oil and knead for a few minutes until you get a ball again. Then add the sugar and repeat.
Once all the sugar has been incorporated, add the egg yolk and mix again.
Finally, add all the salt and knead for 30 seconds.
Place the dough in a graduated container, and mark the level reached. Wrap the container with plastic wrap.
For the bulk rise, keep your dough in a warm place (77˚F/25˚C). With this pleasant temperature, your dough will double in 1 1/2 hours.
When your dough has doubled, unload it to your workspace, and with a bench scraper, start cutting it in small, 75-gram portions.
Form little balls, sprinkle them with water, and then dip them in the sesame seeds, if using.
Place them on a baking sheet 2 inches (5 centimeters) from each other. Repeat until the baking sheet is full, and then cover it with plastic wrap and leave the buns to rise at 77˚F (25˚C) for a maximum of 45 to 50 minutes.
Preheat the oven to 350˚F (180˚C).
Gently brush the leavened buns with milk so that during baking they will acquire a pleasant color.
Bake the buns for 15 minutes, or until done but still tender.
Remove them from the oven and immediately cover them with a cotton towel. Don't let them cool uncovered or they'll become hard.