Preheat the oven to 375 degrees. Wash the potatoes and pierce them with a fork. Bake for about 1 hour or until soft when pierced with the point of a knife. When cool enough to handle, peel the potatoes and place in the bowl of a food processor. Add the butter, sour cream, and maple syrup and pulse until the potatoes are smooth. Season to taste with salt and pepper. Transfer to a casserole, cover, and keep warm until ready to serve.

Melt the butter in a medium saucepan over medium-low heat until pale golden, about 4 to 5 minutes. Add the hazelnuts and cook, stirring, over medium heat until the nuts are golden and fragrant, 2 to 3 minutes. Add the shallot and parsley and cook until just softened, about 1 minute. Season to taste with salt and pepper. If the sauce is to be reheated just before serving, the parsley should be added at this stage to keep it fresh and green. Drizzle the hazelnut butter over the top of the sweet potatoes and serve.