Bring 7 cups of water to a boil in a large (4-qt.), heavy-based saucepan over high heat. Add 1 Tbs. kosher salt and whisk in the cornmeal in a fine stream. Continue to whisk until the polenta begins to thicken, 1 to 3 min. Turn the heat to medium low, and cook, uncovered, stirring frequently with a wooden spoon. (If the polenta becomes too thick to stir, add hot water, a little at a time, until the polenta is soft but will still hold its shape.) After about 20 min. (the polenta should be about three-quarters done), stir in the corn and continue to cook until the corn is tender and the polenta is tender and no longer gritty, 10 to 13 min. more. Stir in the butter and grated Parmigiano. Taste the polenta and add more salt if necessary.