Cook the scrubbed, unpeeled kumara in a covered container in a microwave until just tender. Stand until cook enough to handle, then cut into slices about 1cm thick, removing the skins if you want to. Arrange the slices in a large, flat ovenware dish which has been lightly buttered.
Warm butter until melted. add remaining ingredients and heat until the sugar dissolves, then pour over the kumara. Bake at 180 degrees for 30minutes, or until surface is crisp and golden brown. Serve immediately.