Use a mandoline to make thin, even potato slices to layer in between the sorrel and shallots.
Preheat oven to 350 degrees. Coat an 8-inch square baking dish with 1 tablespoon butter. Arrange shallots in an even layer over bottom of baking dish. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Arrange half of the sorrel over shallots, then layer with half of the potatoes. Season with 1/4 teaspoon salt. Repeat with remaining sorrel and potatoes. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Pour cream over top, and dot with remaining tablespoon butter.
Cover with foil; bake 1 hour. Uncover; bake until potatoes are tender and cream is thick, about 20 minutes more. Let stand 15 minutes before serving.
German Butterball potatoes are an heirloom variety with smooth, golden skin and buttery, deep-yellow flesh. You can find them at farmers' markets or specialty stores.