Heat half the butter and a couple spoons of oil in a pot.
Once hot add all the whole spices and let them sizzle, then immediately add the diced onion and sauté until translucent and soft.
Add the ginger-garlic paste and fry for 1-2 minutes.
Add the chilli peppers, 1 can tomato puree and all the ground spices and sugar. stir and cover and cook for about 20 minutes stirring occasionally. basically let all the flavors meld together and cook down until you start seeing a little oil or butter separating from the mixture.
Add the soaked cashews with the water and about a cup or so of chicken stock or water. Boil with a cover for another 15 mins.
Turn off heat and let the mixture cool enough to be able to blend it into a smooth sauce/ or if you have an immersion blender just blend in the pot directly.
Once blended smooth return the sauce to the pot and bring to a boil, adjust consistency with water/stock.
Dice the grilled chicken pieces and add it to the curry with all the chicken juices and blackened bits.
Stir in the remaining butter and crush dried fenugreek leaves over the top. Cover & simmer for 10-15 mins.
Turn heat to low and add heavy cream to finish the dish. heat through, but don't boil.
Turn off heat and garnish with chopped cilantro.