Season the chicken with 1 teaspoon kosher salt and set aside.
In a Dutch oven or similar large, sturdy pot, melt the butter over medium heat. Once melted, add the onion. Cook until beginning to soften, about 4 minutes. Stir in the garlic, ginger, garam masala, chili powder, cumin, turmeric, and cayenne. Cook, stirring often, until the onion is completely soft and the spices are very fragrant, about 3 minutes more.
Add the chicken pieces, stir to coat with the spices and onion, and cook, stirring often, just until the outsides start to brown, about 4 minutes.
Add the tomato sauce, coconut milk, and remaining 1/2 teaspoon salt. Stir to combine.
Bring the pot to a simmer, then reduce the heat to medium low. Let simmer gently until the sauce thickens slightly and the chicken is tender and cooked through, about 20 minutes.
Uncover and stir in the peas.
Let cool for 3 to 4 minutes, and then stir in the Greek yogurt (do not stir in the Greek yogurt right away or it may curdle). Enjoy warm with rice or naan and a sprinkle of fresh cilantro.
*Garam masala is a blend of many spices and is very important to the final flavor of the recipe. It is available in most grocery stores and can also be found online here.
TO STORE: Leftovers can be stored in an airtight storage container in the refrigerator for up to 5 days.
TO REHEAT: Rewarm chicken tikka masala in a Dutch oven on the stovetop over medium-low heat. You can also reheat this dish in the microwave.
TO FREEZE: Freeze leftovers in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating. The texture of the sauce may change, but the flavors will still be delicious.