Photograph: Martin Poole
Cook the sweet potatoes in boiling salted water for 20-25 minutes until tender, then drain.
Fry the shallots in the olive oil for 7-8 minutes over a high heat, stirring frequently, until golden. Drain on kitchen paper and sprinkle with a little salt.
Mash the sweet potatoes, then mix with the chillies, cumin, butter, milk and a pinch of salt. Reheat and serve scattered with the crispy shallots.