Prepare your curry paste. (step by step instructions provided in the link in the Recipe Notes below)
Peel, wash and dice the sweet potato, and then set aside.
Add coconut oil and butter to a dutch oven on medium heat. When the butter is melted, add the curry paste and fry for 5 - 6 minutes, stirring constantly, until it starts to stick to the pan.
Add coconut milk which should lower the heat in the pan and stir in thoroughly to prevent the curry from burning. Add the sweet potato and roasted/re-roasted peanuts. Cover and allow to simmer on low to medium heat for 30 - 40 mins, stirring occasionally to prevent sticking to bottom of pan.
Check that sweet potato is soft, then transfer to blender. Blend to a puree and then return the puree to the pot. Add 100mls of water, sugar, fish sauce, and salt and pepper to taste. Simmer for 5 minutes.