Soak the almonds and cashews in water overnight, or for at least 8 hours.

Cut the chicken thighs into roughly 1-inch cubes.

Combine the chicken with yogurt, olive oil, 1 tablespoon ginger garlic paste, 1 teaspoon ground coriander seed, 1 teaspoon Kashmiri chili powder, cumin, and salt in a large bowl.

Cover the chicken marinade and refrigerate it for at least 60 minutes or up to overnight.

Heat the skillet over medium-high heat and cook the marinated chicken in batches until browned and cooked through.

Melt 1 1/2 tablespoons of the butter in a saucepan over medium heat.

Add the remaining 1 tablespoon ginger garlic paste and sugar, then sauté until fragrant.

Stir the diced tomatoes into the saucepan and cook for 8 to 10 minutes until most excess liquid has evaporated.

Add the remaining 2 tablespoons ground coriander seed, garam masala, remaining 1/2 teaspoon Kashmiri chili powder, and remaining salt.

Cook the tomato spice mixture for 2 to 3 minutes, stirring frequently.

Transfer the tomato mixture to a blender and add the drained soaked almonds and cashews.

Blend the mixture until completely smooth and creamy.

Return the blended sauce to the saucepan and stir in 1 cup water until smooth.

Bring the sauce to a gentle boil over medium heat.

Add the cooked chicken and heavy cream to the boiling sauce, then stir until evenly coated.

Cover the saucepan and simmer the butter chicken for 5 minutes over low heat.

Turn off the heat and stir in the remaining 1/2 tablespoon butter and dried fenugreek leaves.

Cover the butter chicken and let it rest for 10 minutes so the flavors infuse.

Serve the butter chicken over saffron rice.