Melt 2 tablespoons of the butter with 2 tablespoons of the oil in a large skillet over medium-high heat, add the pumpkin cubes, and saute, stirring frequently, until tender, about 10 minutes; set aside.
In a medium-sized heavy saucepan, heat the remaining oil over medium-high heat. Add the onion and garlic, and saute, stirring continuously, just until softened, 3 to 4 minutes. Add the rice and continue to stir, using a wooden spoon, to coat the rice with the oil.
Add the wine and continue cooking, stirring often, until it has been absorbed by the rice. Ladle in enough of the hot broth to cover the rice completely, about 3 cups. Adjust the heat to maintain a gentle simmer and continue to cook, stirring constantly, until all the liquid has been absorbed, 10 to 15 minutes.
Ladle in 1 cup more of the remaining broth and cook, stirring constantly until liquid has been absorbed. Repeat with 1 cup more. Add the remaining broth and cook, stirring constantly, until the rice is al- dente, and most of the liquid has been absorbed.
Stir in the diced pumpkin and reduce the heat to very low so that the risotto no longer simmers. Stir in the Parmesan and remaining butter to give the risotto a nice, creamy finish. Spoon it immediately onto serving dishes. Garnish with additional grated cheese and serve immediately.