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- Steam sweet potatoes until soft and cooked. Remove the skin and, while they are still hot, blend in a food processor into a puree.
- Combine sweet potato puree, sugar, sifted flours and double-action baking powder in a large bowl of an electric mixer fitted with a dough hook. Mix well until a dough is formed.
- Turn dough onto a lightly floured tabletop and knead until the dough is soft and evenly textured. Shape into a ball.
- Roll dough into 0.5cm thickness. Cut out into a big piece of rectangle (28x30cm). Brush lightly with corn oil and roll up - Swiss-roll style - and cut into sizable portions of 5cm thickness.
- Gather the remaining dough and repeat the process.
- Place each individual mantou on a small piece of cut-out greaseproof paper. Steam over rapid boiling water over high heat for 10-15 minutes. Remove and serve hot.