For the dough: In the bowl of a stand mixer with the whisk attachment, whisk the egg yolks, whole egg, sugar, butter and buttermilk. Add approximately 2 cups of the flour along with the yeast and salt; whisk until moistened and combined. Remove the whisk attachment and replace with a dough hook. Add all but 3/4 cup of the remaining flour and knead on low speed for 5 minutes. Check the consistency of the dough and add more flour if necessary; the dough should feel soft and moist but not sticky. Knead on low speed 5 minutes more or until the dough clears the sides of the bowl. Turn the dough out onto a lightly floured work surface; knead by hand about 30 seconds. Lightly oil a large bowl. Transfer the dough to the bowl, lightly oil the top of the dough, cover and let double in volume, 2 to 2 1/2 hours.