Spread each slice of bread with 1 tsp of butter. Cut buttered bread into 1/2" cubes. Combine sugar and cinnamon. Arrange half of rhubarb in bottom of greased 8" baking dish. Top with half of bread cubes and half of sugar mixture. Repeat. Dot with remaining butter. Cover and bake in 375F oven for 20 minutes, uncover and bake 20 to 25 minutes more or till lightly browned.