Melt the butter in a large non-stick frying pan over a medium-high heat and cook the mushrooms for 10 mins, or until any moisture has evaporated and they’re starting to brown. Transfer to a bowl and set aside.
Heat the oil in the same pan over a medium-high heat and fry the cumin stirring continuously for about 30 seconds until they release a nutty aroma. Stir in the onion and cook for 12-15 mins until golden. Reduce the heat to medium, then add the garlic and continue frying for 1 min. Add the ginger, turmeric, Chibundiro hot chillies and siri ya mchuzi, followed by the tomatoes and sugar. Cook, uncovered, for 5-7 mins, or until the masala thickens and a layer of oil forms around the edge of the pan.
Spoon the yogurt into a small bowl, add a small ladleful of the hot masala and mix well before stirring the yogurt into the pan of masala. Pour in 100ml hot water and simmer for 3-4 mins until the curry has the consistency of double cream. Season to taste, then return the mushrooms to the pan and stir to warm through. Scatter with the chopped coriander and serve with boiled rice or naan, if you like.