Bring a pot with 2 quarts (2 L) of water to a boil. Add the shrimp, blanch 30 seconds, then drain.
In a large, heavy saucepan, melt 4 tablespoons (60g) of the butter. Add the onion and cook over medium-low heat until it is translucent. Stir in the rice. Cook, stirring, about 5 minutes, until the rice is well coated with the butter and begins to whiten. Have the stock barely simmering in another saucepan. Add the wine to the rice, stir, then add 2 ladles of the hot stock.
Cook 3 minutes.
Stir in the radicchio.
Continue adding stock, a couple of ladles at a time, stirring and adding more as the stock is absorbed by the rice.
Regulate the heat so the rice and stock become somewhat tender but still a bit hard in the middle. There should still be some stock left at this point.
Add all but 4 of the shrimp.
Stir, then continue adding stock for another few minutes, as needed, until the rice is plump, offers only the slightest resistance to the bite, and there is just enough thickened, creamy, sauce-like broth to moisten the rice.
Stir in the cognac and remaining butter.
Season with salt and pepper. Pour the risotto into plates, garnish each serving with a shrimp, top with a bit of parsley, and serve at once.