Heat a small dry stainless-steel skillet or saucepan over medium-high heat. Add the carom, cumin, and coriander and toast until they turn fragrant and light brown, 30 to 45 seconds. Remove from the heat and transfer to a plate to cool completely. Once cooled, add the spices to a coffee grinder or spice mill along with the remaining ingredients except the kala namak. Grind to a fine powder. Transfer to an airtight container and store in a cool dark place for up to 6 months. When ready to use, add ¼ tsp ground kala namak to every 1 tsp of chaat masala.