Add sweet potato to a pot. Pour milk in until it just covers the potatoes. Boil until fork tender. Drain and set aside to cool.
Warm the milk to about 108℉. Add the yeast and sugar, mix and let it sit to bloom for 10 mins.
Place the flour to the bowl of a stand mixer , add salt and mix. Add in the bloomed yeast and the egg.
Rice the sweet potato directly into the bowl. With the dough hook attachment, start the mixer on medium speed.
Once the dough comes together, start slowly adding the softened butter 1 tbsp at a time. Making sure to the butter is fully incorporated before adding the next.
Once all the butter incorporated, continue to mix for 5-7 mins until the dough starts to detach from the side of the bowl.
Knead the dough into a ball, place into an oil bowl, cover and let it rise until it doubles in size (1 1/2 - 2 hrs).
Punch the dough down, cut the dough into 12 equal pieces that weigh about 100-105g each.
Place parchment paper into a 9x12 baking dish. Add the dough balls to the baking dish. Cover and let it rise while the oven is heating.