In a 4-quart saucepan, melt 4 tablespoons of the butter over medium heat. Add the stock and bring to a simmer. Season with salt and pepper.
Slowly whisk in the polenta. Reduce the heat to low and cook, stirring frequently, until the polenta is hydrated, about 2 hours.
Remove from the heat and whisk in the mascarpone, Parmesan and remaining 2 tablespoons butter. Season to taste with salt and pepper and serve topped with a drizzle of olive oil and more Parmesan, if desired.