Preheat oven to 350 degrees F. Melt 6 tablespoons butter. In a large bowl, stir together melted butter, sweet potatoes, cream, 1/2 cup brown sugar, cinnamon and nutmeg until the mixture is smooth. Season with salt to taste. Transfer mixture to a 2- to 2 1/2-qt baking dish

Melt remaining 4 tablespoons butter. In a medium bowl, stir together remaining 1/2 cup brown sugar and flour. Stir in hazelnuts and melted butter until crumbly and moist.

Sprinkle topping over the casserole. Bake 20 minutes, until topping is browned and bubbly.