Coat 1-quart ceramic gratin or baking dish with butter and sprinkle with 1 tablespoon sugar. (You will be serving clafoutis from this dish.) Pour half of batter in dish and distribute 3/4 of plums evenly on top. Pour in remaining batter and scatter last of plums on top. (Don't worry if they start to sink.) Sprinkle with remaining 1 tablespoon sugar.