Use sharp poultry shears to cut the chicken into smaller pieces. Alternatively, you can use a mix of bone-in chicken pieces like thighs, drumsticks and breasts chopped into manageable pieces.
Place the chicken pieces in a large mixing and toss with ⅓ of the minced garlic, ⅓ of the minced shallots, ¾ teaspoons salt and all of the curry powder and turmeric powder. Marinate for about 10-15 minutes.
Place a heavy bottomed pot over medium heat and heat 2 tablespoon of oil. If using the annatto seeds, add them to the oil and let stand for a few seconds until the oil turns to a reddish color. Discard the seeds.
Add lemongrass stalks and ginger and stir and cook for about 1 minute until fragrant. Then add the remaining minced shallots and garlic, cook for another 1-2 minutes until fragrant.
Slightly increase the heat and add the chicken pieces to the pot. Cook until the outside is no longer pink, stirring a few times. This may take about 10 minutes.
Add the coconut milk (reserving ¼ cup) and water to the pot and bring to a simmer. Then add the sweet potatoes, curry leaves (or bay leaves), remaining salt and sugar. The liquid should be enough to almost cover the chicken and sweet potatoes.
Adjust the heat to maintain a simmer. Cover the pot and simmer for 25-30 minutes or until the chicken and sweet potatoes are tender.
Once the chicken and sweet potatoes are tender, add the fish sauce. Taste and adjust seasonings to your liking. Stir in the remaining coconut milk. If you are serving the curry with vermicelli noodles, you can add some water to thin it out.