Place yucca pieces in a saucepan and cover with water. Bring to a boil and cook until very tender and soft.
Remove yucca from saucepan, place it in a bowl and mash it with a potato masher until smooth. If necessary add a small amount of milk. Add butter and season to taste, mixing thoroughly by kneading with both hands. Add cheeses and combine.
Transfer puree to a serving heatproof mold and top with butter.
Heat and broil in oven before serving.
Garnish with chopped chives or parsley.