In a medium saucepan combine milk, water, salt and corn. Set over med-high heat and bring to boil. Watch carefully! (I have a tendency to boil this over which creates a huge mess on the stove!)
Slowly pour polenta into boiling liquid with one hand, while whisking the mixture with the other. Continue whisking for a couple minutes to minimize clumping.
Reduce heat to low and continue stirring (using a wooden spoon) until polenta is very thick. The timing of this stage will depend on the polenta grind. A quick cooking polenta will take only 4-6 minutes, while a medium grind polenta may take 10-15 minutes. For added creaminess continue cooking an additional 15 minutes, 30 minutes total for a medium grind cornmeal. If while cooking, polenta begins to look dry or too thick, you can stir in some warm water (about 1/4 cup at a time) to desired thickness.
Stir in butter and a couple tablespoons of parmigiano-reggiano and serve immediately.