Take a bowl and add mashed sweet potato, flour and mix well.
Now add cumin, baking soda, salt, sugar, mix well.
Finally add curd and prepare dough. Knead the dough well.
Keep this in warm place for 8 to 10 hours for resting or overnight (in the morning refrigerate the dough if not using it immediately)
Knead once, make small to medium round balls and roll into thick poories.
Heat oil on medium flame in a kadai, drop slowly the poories and fry on medium flame until done (they turn golden to brown colour)
Serve hot sweet potato mangalore buns with coconut chutney or sambhar