The spiciness of the masala is directly related to the type of chili you use and the amount of capsaicin it contains. If you want to mellow out the heat, you can add a bit of heavy cream, malai, or dried milk powder to the mix - dairy helps to block capsaicin (the compound that tastes "spicy") from your taste receptors. I've also found that adding a few spoonfuls of mashed potatoes can help to mitigate the heat, though it'll change the texture and appearance of the curry a bit.