Brush the inside of a 9x13-inch baking dish with a tablespoon of the olive oil.
In a medium saucepan, bring 7 cups of water to a boil. Add the salt, and then whisk in the polenta in a steady stream. Lower the heat so that the polenta is simmering and cook, stirring frequently with a wooden spoon to prevent clumping, until the mixture is thick and creamy, and the polenta is cooked through. (This should take 15 to 20 minutes -- if it thickens too much, just add a little more water to the pot.)
Stir in the mascarpone and Parmesan, along with several grinds of pepper if you're using it.
Stir the mascarpone and Parmesan into the polenta until thoroughly combined. Working quickly, pour the polenta into the baking dish and spread it out in an even layer. Cover the dish and refrigerate for at least a couple of hours, until firm.
Place an oven rack about 5 inches from the heat source and turn on your broiler. Using a sharp knife, cut the polenta into 1-inch squares and arrange them on a baking sheet. Brush the tops of the squares lightly with olive oil. Broil the polenta squares for a minute or two, keeping a close eye on them, until they are toasty and brown. Let them cool on the baking sheet for a minute and then serve, drizzled with a little more olive oil if you like.