Preheat oven to 425°. Cook 1/4 teaspoon caraway seeds in a small saucepan over low heat 1 minute or until toasted. Add butter, and cook until melted; set aside.
Spread rock salt evenly in the bottom of a shallow 3-quart casserole. Sprinkle 1 teaspoon caraway seeds, cumin seeds, and pepper over rock salt. Arrange the potatoes over salt mixture. Bake at 425° for 35 minutes or until tender. Remove potatoes from salt mixture; discard salt mixture. Combine the butter mixture and potatoes; toss to coat.