6 tablespoons butter or margarine, cut into chunks
Salt and pepper
How to Make It
Put hazelnuts in a shallow 3-quart casserole (about 9 by 13 in.). Bake in a 350° oven until golden under skins, about 15 minutes. Pour onto a towel and let stand until cool enough to touch, about 5 minutes. Rub nuts in towel to remove loose skins. Lift nuts from towel; discard skins. Whirl nuts in a blender or food processor until finely chopped.
Meanwhile, in a bowl, blend sour cream, chicken broth, and marjoram until smooth.
Scrub and peel potatoes. Cut crosswise into 1/8- to 1/4-inch slices. Cover bottom of casserole with about 1/4 of the potato slices, overlapping slightly to make an even layer. Pour about 1/4 of the sour cream mixture evenly over potatoes to cover. Repeat to layer remaining potatoes and sour cream mixture, ending with sour cream. Cover pan tightly with foil; set in a 10- by 15-inch baking pan.
Bake in a 400° oven until potatoes are tender when pierced, 1 to 1 1/4 hours.
While potatoes bake, in a bowl combine chopped hazelnuts, bread crumbs, cheese, and butter. Rub with your fingers until butter is incorporated and mixture forms crumbly chunks. Uncover potatoes and sprinkle evenly with hazelnut streusel. Bake until streusel is browned, 8 to 10 minutes longer. Add salt and pepper to taste.