Preheat oven to 350 degrees. Grease a 9 inch square pan. Pour chopped rhubarb in the bottom.
In a bowl combine flour, granulated sugar, milk, melted butter, baking powder, salt and vanilla extract. Stir until smooth. Pour over rhubarb and spread evenly.
In another bowl, combine powdered sugar and cornstarch. Scoop mixture evenly over top of rhubarb and batter mixture. Pour water over top.
Bake for 1 hour. Remove from oven, cool and serve.
Top with whipped cream or ice cream. Refrigerate leftovers.