Bring the water to the boil in a large pan, season with salt. Combine the two types of polenta, gradually pour into the boiling water, stirring constantly. Simmer for min. 1 hr. over a very low heat, stirring constantly to form a thick paste. The longer polenta is left to cook, the better it tastes. If the polenta becomes too firm, add a little boiling water. Stir in the butter and cheese until the cheese melts.