In a large pot, bring one-liter vegetable stock to the boil. Stir in the polenta, lower the heat to minimum, add the lid to the pot and let it simmer for one and a half hour.
Stir occasionally and add more water during cooking when your polenta gets too dry. You will need about 700ml of water, but it depends a little on the polenta you use.
After 90 minutes, stir in the butter and grated parmesan until melted and well combined.