Preheat oven to 400 degrees F (200 degrees C). Lightly grease a 9 inch square baking dish.
In a saucepan, mix sugar and cornstarch for filling. Stir in the rhubarb and water. Bring to a boil. Cook and stir for 1 minute. Transfer to the prepared baking dish. Dot with butter, and sprinkle with cinnamon.
In a medium bowl, sift together flour, sugar, baking powder, and salt for topping. Cut in the butter until the mixture resembles coarse crumbs.
In a small bowl, mix together milk and egg. Add all at once to dry ingredients, stirring just to moisten. Drop by teaspoonfuls on top of the rhubarb mixture. Sprinkle with sugar.
Bake for 20 minutes in the preheated oven, until crisp and lightly browned.