How to Make It
In a medium saucepan, bring 2 cups of water to a boil over moderately high heat. Add the butter and salt, then slowly whisk in the polenta. Reduce the heat to low and cook, whisking, until the polenta has somewhat thickened, about 20 minutes. Remove from the heat; let cool for 2 minutes. Whisk in 1/4 cup of the mascarpone.
Spoon 2 tablespoons of the polenta onto a foil-lined baking sheet and, using the back of a spoon, spread it into a 3-inch round pancake about 1/4 inch thick. Repeat with the remaining polenta, forming 12 pancakes. Let stand at room temperature for 10 minutes.
Preheat the broiler. Broil the pancakes about 6 inches from the heat for 3 minutes, or until slightly dry on top. Overlap 3 pancakes on each of 4 warmed plates and top with the peaches and 1 tablespoon of mascarpone. Serve immediately and pass the sugar separately.