Halve and deseed the chillies. Place in a large non-stick frying pan on a high heat with the tomatoes and blacken all over, turning occasionally. Meanwhile, peel the garlic and ginger, and finely grate into a large bowl. Add most of the garam masala, a pinch of sea salt and black pepper and 1 tablespoon of yoghurt. Deeply score the chicken breasts at 1cm intervals, then massage with the marinade.
Once charred, remove the tomatoes and chillies to a board, returning the pan to a medium heat with ½ a tablespoon of olive oil and the chicken. Cook and char for 10 minutes, turning halfway, while you pinch off and discard the tomato skins and roughly chop 1–2 of the chillies, to taste. Remove the gnarly chicken from the pan and go in with the tomatoes, chopped chillies and cashew butter. Pour in 250ml of boiling kettle water and stir to pick up the sticky bits. Let it bubble vigorously for 2 minutes and once it starts to thicken, return the chicken to the pan, turning in the sauce for a final 2 minutes, or until cooked through, then remove to a board. Off the heat, season the sauce to perfection, then ripple through the remaining yoghurt. Slice the chicken and serve with the remaining chilli and garam masala.
If you want to take the edge off the chilli, I find adding a teaspoon or two of mango chutney to the sauce works a treat. That little bit of extra sweetness helps to balance the heat.