Combine the cream, milk and salt in a heavy-bottomed stainless pot. Slowly bring the mixture to a boil over high heat, stirring often with a heat-resistant rubber spatula so the milk/cream doesn't burn the bottom.
Reduce the heat to medium and slowly rain in the polenta – use a wire whisk to stir the polenta to make sure there are no lumps. Switch out the whisk and stir with a wooden spoon, then reduce the heat to the lowest setting on your stove (barely simmering) and cook, covered, for 1 hour, stirring every 10 or 15 minutes so the bottom doesn't burn. If you don't have time to babysit the polenta, you can at this point transfer the mixture to a slow cooker and place the setting on the lowest temperature for 3 hours.
If the polenta seems dry, boil 1/2 to 1 cup more milk and stir it into the polenta until it's loose and creamy. Turn off the heat and add the butter and cheese. Mix just before serving and add an extra sprinkle of cheese on the plate if desired.