Cook the potato using your method of choice (boil, steam, or bake) until tender.Peel, mash and measure 3 oz (about 1/2 cup). Allow cooling to room temperature. (About 70F)
Melt the butter, set aside and allow to cool from hot to warm. (should be 115F)
In a bowl of a standing mixer, sift the flour, yeast, salt, nutmeg and baking soda and stir to combine.Add the sugar.
Add the milk, melted butter, cooked potato and egg then mix using the dough hook until the dough is smooth.
Place the dough in a large greased bowl, cover with plastic bag and allow to rest until double in size. About 90 minutes.
Turn dough onto a lightly floured surface and dived it into 16 small pieces (about 40g each).
Roll each piece into a small ball then flatten it. Using your thumb and forefinger gently pinch it in the center to create the hole and stretch it into 3" round.
Place aside for the second rise about 75-90 minutes.
Place the oil in a large cooking pan and warm it over the stove, medium-high heat until it reaches 350F.
Gently dump the donuts and fry for about 90 seconds, turning sides half way throug
Remove from oil and place in the bowl of sugar, cover the donut with sugar.