Steam the cleaned sweet potatoes until soft. Mash while still hot, then add to the glutinous rice flour. Add sugar and salt and mix until it turns crumbly.
Add oil and pour in hot water gradually. Knead into a soft dough. Cover the dough with a clean kitchen towel and leave aside.
Steam the soaked mung beans for 20 minutes or until the peas are soft. Remove the steamed mung beans and place into a food processor. Add sugar, salt and oil. Process into a fine paste then remove and shape into small round balls, to be used as the filling.
Shape small balls of skin dough into rounds. Flatten each piece and place a small ball of mung bean filling in the centre. Wrap up to conceal the filling and seal.
Dust lightly with extra glutinous rice flour. Press the balls into a wooden angkoo mould. Knock out the angkoo and place on a banana leaf.
Steam over medium heat for 8-10 minutes. Brush the steamed angkoo lightly with oil.