With the skin on, cut the kumara into 3 or 4 large chunks. Place in a large saucepan and cover with water. Bring to a boil then simmer until almost tender. Drain in a colander set in the sink and leave there to cool.
When cool, peel off the skin of the kumara, cut into smaller chunks then place in a baking dish. Preheat the oven to 180 degrees C.
In a small saucepan, combine the butter and brown sugar. bring to a boil and stir until it caramelises. Pour over the kumara then place in the oven for about 20 minutes. Serve.