Preheat oven to 400 degrees. Lightly brush 6 standard muffin cups with butter. Thinly slice potatoes. Place 2 slices in each cup and season with salt and pepper. Continue adding potatoes, seasoning every few slices, until cups are filled. Pour 1 tablespoon heavy cream over each. Bake until potatoes are golden brown and tender when pierced with a knife, 30 to 35 minutes. Run a thin knife around each gratin. Place a baking sheet or large plate over pan and invert to release gratins. Flip right side up and serve.