This beautiful risotto dish calls for two kinds of clams over a rich, subtly lemony Arborio rice base.
Heat a Dutch oven over medium heat. Add the olive oil and pancetta and cook, stirring occasionally, until the pancetta is golden brown and crispy, about 5 minutes. Using a slotted spoon, remove the pancetta to a paper-towel-lined plate and reserve.
To the pan with the pancetta drippings, add the shallots, garlic and salt. Cook until fragrant, stirring with a wooden spoon, 2 minutes. Stir in the rice and coat with all the flavors of the pan. Toast the rice for 2 minutes. Deglaze with 1 cup of the prosecco and cook, stirring constantly, until the liquid is almost entirely absorbed. Add 1 cup of the clam juice and cook, stirring, until almost completely absorbed, 6 to 8 minutes. Continue adding the clam juice, 1/2 cup at a time, stirring often and allowing each addition to be absorbed before adding the next. Cook until the rice is tender but not mushy, about 20 minutes.
Meanwhile, to a small saucepan, add the remaining 1/2 cup prosecco and the New Zealand clams. Cover the pan and place over high heat. Cook, shaking occasionally, until the clams have opened, about 4 minutes. Discard any clams that do not open. Set aside.
To finish the risotto, stir in the Parmesan, butter, reserved pancetta and the chopped littleneck clams along with their juice. Cook until the clams are cooked through and the butter and cheese are melted in, an additional 2 minutes. The risotto should have some movement and not be too thick.
In a small bowl, mix together the parsley and lemon zest. Stir half of the mixture into the risotto. Using a slotted spoon, scoop the steamed New Zealand clams over the top of the risotto and finish with the remaining parsley mixture.