You'll know it's done when the outsides are golden brown and crispy. Butter mochis are very forgiving and are very difficult to overcook. Once they are golden brown, the insides will be cooked all the way through as well, but make sure you check at the 30 minutes mark to prevent it from burning. To check if they are underdone, you can use a toothpick and insert it in the middle of a few mochis, if it comes out clean then the center has been fully cooked.