Add the chicken, tomato, and the paste ingredients, mixing well to coat the chicken. Continue stirring until the chicken is evenly cooked and the tomato has broken down. Add the cubed potato, and cover and simmer until the potato is cooked and the gravy has thickened, about 30 minutes. Stir in the coconut milk, and heat for a few minutes before. Do not cover the pan when using coconut milk, as the condensation will fall back into the pan and may cause curdling.
Coconut milk is available in either canned or dried powder form in most Asian market stores.
Mix according to package directions.