Heat 2tbsp of the oil in a sauté pan or casserole wide enough to hold the chicken pieces in a single layer. Add the chopped rosemary and then the chicken thighs, skin side down.
Sprinkle with sea salt and freshly ground black pepper, and cook over a medium heat until lightly browned. Then turn over the pieces and season again, then cook until the other side is lightly browned.
Add the whole garlic cloves. When they have only just begun to colour on one side, add the butter, if using – it gives a lovely creaminess to the sauce – and pour in the Muscat wine.
Put a lid on and simmer over a low heat for about 25 minutes, until the chicken thighs are tender, moist and juicy, turning them over at least once and removing the lid towards the end to reduce the sauce. Taste for seasoning – you need enough salt and pepper to balance the sweetness of the wine.
At the same time, heat the remaining 2tbsp oil in a wide frying pan and add the grapes. Cook over a medium heat for about 20 minutes, shaking the pan and turning the grapes over until they have collapsed and their juice is much reduced. Pour them over the chicken pieces and heat through together.