Heat milk and water in a thick-bottomed pot over high heat. When the liquid comes to a boil, add salt, then pour polenta in gradually while continuously whisking. Continue to whisk for about 4 to 5 minutes until the polenta is set (which means when you stir the polenta it doesn't settle back to the bottom of the pot; instead you can see it suspended evenly throughout the liquid). At this point immediately turn the heat down to low, so the polenta bubbles only slightly. Then slice the butter thinly and place over the top of the polenta so it melts, forming a thin layer on top.

Traditionally polenta is stirred for about 45-60 minutes over low heat with a wooden spoon, until it comes away from the sides of the pan. If you have other things to do, you can put the flame down very, very low and cook the polenta for 75 to 90 minutes without stirring it.