Step 1 In a large bowl, stir together the potato flour, whole wheat flour, yeast, salt, sugar and one-half cup of the flour. Add the warm buttermilk, melted butter and egg. Beat at low speed for 30 seconds. Scrape down the sides and bottom of the bowl, then beat at high speed for 3 minutes. Reduce the speed to low. Gradually add the remaining flour, about one-half cup at a time, scraping the bowl between additions. The dough will come away from the sides of the bowl.
Step 2 Turn out onto a lightly floured surface (if there are any dry bits at the bottom of the bowl, discard them). Knead the dough for a couple of minutes until smooth and elastic; if it's too sticky, sprinkle flour as needed.
Step 3Place in a greased bowl. Turn over to grease the top, and cover. Let rise until double in size, about 1 to 1 1/2 hours.
Step 4At this point, you can refrigerate the dough until you are ready to bake. First, deflate the dough by pressing down the middle and pulling the sides toward the center. Turn it over in the bowl, then brush the top with melted butter or press a piece of plastic wrap against the surface to prevent a crust from forming. Refrigerate, tightly covered, for up to 48 hours. Punch down and let stand at room temperature for about an hour. Proceed with the next step at least 2 hours before serving.
Step 5Line a baking sheet or large baking pan with parchment paper or a silicone mat. Turn the dough out onto a lightly floured surface. Pat it into an 8-inch square, about an inch high. Using a sharp knife and a downward motion, cut the dough into 16 (2-inch) squares. Form each square into a ball, then pull down the sides to create smooth tops and firmly tuck the edges underneath.
Step 6Arrange the rolls on the lined sheet with the sides almost touching. Cover loosely with plastic wrap and allow to rise until double in size, about 30 to 45 minutes. They should look like puffy cushions.
Step 7Heat the oven to 350 degrees. In a small bowl, beat together the egg yolk and 1 tablespoon water to make a glaze. Gently but thoroughly brush the tops of the rolls (you will have some glaze left over). Bake until golden brown, about 20 minutes. Serve warm, or cool on a rack.