Preheat your oven to 170 degrees F. Heat the milk in a medium pot over low heat until it reaches 100 degrees F. Remove pot from heat and add in the yeast and 1/2 teaspoon sugar. Do not stir. Let sit until foamy, which should take roughly 5-10 minutes. (If the mixture doesn't foam you may need new yeast or you could have killed the yeast with milk that was too hot. Start over or your rolls won't rise).
While the yeast is sitting, melt the butter and let it return to room temperature. Once cooled, whisk in the vanilla extract and egg yolk until smooth. Whisk this mixture into the yeast/milk mixture.
Using a stand mixer, mix together the flour, the remaining 1/3 cup sugar, the salt and the cinnamon. Once combined, make an indent in the center and pour the yeast mixture into the indent. Mix on low speed with the dough hook until thick and slightly sticky (~2 minutes), scraping down the sides with a spatula as needed. Knead on medium speed until the dough gathers around the hook, (~5-6 minutes). If the dough is not gathering around the hook you can slowly add additional flour (1/4 cup up to 3/4 cup extra). You DO want the dough to be slightly sticky and tacky. Resist the urge to add too much additional flour as this will take away from the softness of the dough, but the dough should gather around the dough hook and be slightly sticky and tacky, but not stick to your fingers.
Remove the dough and shape into a ball (it will be a little sticky!). Grease the mixing bowl with cooking spray or butter and return the dough to the bowl, turning to coat with the spray or butter. Cover with plastic wrap or a towel and let rise until doubled. This takes roughly 45 mins to 1.5 hours. Placing the dough in a warm environment will make it rise faster. (I turn off the oven that preheated to 170 degrees, add the bowl of dough into the oven, and crack the oven open a 4-5 inches).
Lightly flour a clean surface and roll out the dough. Try to get a rectangle shape that is approximately 15 x 12 inches (with the 15 inch side facing you). Avoid overworking the dough.