Peel sweet potato (or potatoes – you just want about 12 ounces), chop into chunks and place in a pot with cold water. Bring to a boil and and cook until potatoes are fork tender, about 20-25 minutes. Drain potatoes, then place in a bowl and mash.
In the bowl of an electric mixer, combine water, yeast, 2 tablespoons honey and olive oil. Mix with a spoon and let sit until foamy, about 15 minutes.
Once foamy, add potatoes, remaining honey, 4 tablespoons melted butter, buttermilk and salt. Gradually add flour with the mixer on low until a sticky dough forms, then knead with the dough hook for about 2-3 minutes. Brush a large bowl with melted butter, then place the dough in the bowl. Cover and let rise in a warm place for about 2 hours.
Using a bit of flour, knead dough until silky then divide in half. Roll into a long rope, then cut off pieces of dough (about 2 ounces each) and roll into a bowl. Place them into a greased 9×13 baking dish – you should be able to fit about 15 in there (using 2 pans). If desired, you can freeze dough at this point for up to 2 months. If baking immediately, cover the baking dish and let sit in a warm place to rise for about 30-45 more minutes.
Preheat oven to 375 degrees. Brush rolls with beaten egg, then bake for 20-25 minutes, or until the tops are slightly golden brown. (If you want to bake both pans at once, set one on the lower rack and one on the upper rack in the oven, then switch half way through baking.) Let cool for a few minutes, then serve warm with butter and honey.