Cook the sweet potatoes in a saucepan of boiling water for about 15 minutes until tender. Drain, reserving 5 tablespoons of the cooking water. Return the potato to the pan and set over a low heat to dry off the excess water, then mash with the oil.
Place the reserved cooking water in a jug and, while still warm (but not hot), add the yeast and stir to dissolve. Leave in a warm place for about 10 minutes until bubbles appear on the surface.
Stir the yeast mixture into the potato, then gradually stir in the remaining ingredients to form a dough. Tip the dough out on to a lightly floured surface and knead for 5 minutes until smooth, then place in a lightly oiled bowl, cover with oiled clingfilm and leave to rise in a warm place for 45 minutes–1 hour.
Knock back the dough and then divide into 8 pieces. Shape into balls, then flatten slightly. Transfer to a lightly oiled baking sheet and cover loosely with oiled clingfilm. Leave to prove for 30 minutes.
Bake in a preheated oven 220°C (425°F), Gas Mark 7, for 15–20 minutes until risen and golden brown and hollow when tapped on the base. Transfer to a wire rack and leave to cool for 5 minutes before serving.