Preheat oven to 425°F. Brush an 8 1/2-inch tapas pan with 2 Tbs. butter. Place 1 potato slice in center of pan and cover bottom with concentric circles of potato slices, slightly overlapping. Line sides of pan with potato slices, slightly overlapping. Season with salt and pepper; drizzle with 1 Tbs. butter. Repeat with remaining ingredients to form about 4 layers.

Cover pan tightly with a piece of buttered aluminum foil. Set pan over medium heat; cook until potatoes sizzle, 1-2 min. Bake for 30 min. Remove foil; press potatoes with back of a spatula. Bake until golden brown, about 30 min. more.

Run a knife around inside of pan. Press a plate against potatoes and carefully tilt pan over a bowl to catch excess butter. Invert potato cake onto plate and cut into 6 wedges. Serve immediately.