Wash the yams and prick with a fork. Place on a baking tray and bake at 400°F for 1 hour, or until soft when gently squeezed.
Cut in half lengthwise and carefully scoop the flesh out into a bowl, being careful not to break the skins. You may find it helpful to hold the yams in a cloth. Add the butter and mash well. Add the thyme, chives, salt, and pepper to taste and mix well.
3. Spoon the mixture back into the skins, piling up slightly in the center. Mix grated cheese and chili garlic sauce. Then top yams with this and return to the oven for about 10 minutes, or until the cheese is melted and the yams are heated through.