Place the sliced rhubarb and water in a medium saucepan. Mix the sugar and cornstarch together and add this to the rhubarb mixture. Bring this to a boil, and cook for 1 minute. Pour out into a greased 13x9 inch baking pan, and dot with butter. Sprinkle with cinnamon as desired.
For the Cobbler Topping
Sift the flour, sugar, baking powder, and salt together. Cut in the butter until it looks like coarse crumbs. Mix the milk and egg in a small mixing bowl, and pour this over the flour mixture. Stir ingredients to moisten them, but do not overmix. Drop by spoonfuls over the top of the cooked rhubarb. Sprinkle with sugar. Bake in a preheated 400 degree F oven for about 20 minutes, until the cobbler biscuit topping is browned and done, and the rhubarb is bubbly.