In a medium size heavy saucepan, with a whisk combine the stock, milk, salt and polenta and bring the mixture to a boil stirring frequently.
Reduce the heat to low. Switch to a wooden spoon or paddle and continue to stir every 1 or 2 minutes for about 20 minutes, or until the mixture comes away from the side of the pan and the grains of polenta begin to soften. The mixture will be very thick and the spoon will be able to stand in it for a few seconds.
Add 2 Tablespoons of butter, Parmesan and freshly ground black pepper, stirring until they are thoroughly mixed in.
Remove from the heat.
Spray an 8 X 12 pan with vegetable spray. Mound the polenta in the pan and then using a spatula spread the polenta out into an even layer. You may have to dip the spatula into hot water so that it does not stick to the polenta.
Cover with a towel -- do not use plastic wrap because it will sweat. Let the polenta rest at room temperature until cool. Put in the refrigerator to chill.
When ready to serve preheat an indoor broiler or an outdoor grill to high heat.
Melt the remaining 2 Tablespoons butter. Cut the polenta into squares and brush with melted butter.
Grill about 3 inches from heat for 4 minutes on each side, or until lightly toasted.
Remove from heat, and put on cooling rack. Serve with desired topping.