Preheat oven to 350 degrees.
Grease a 9 inch square pan.
Pour chopped rhubarb in the bottom.
In a bowl combine flour, nutmeg, granulated sugar, milk, melted butter, baking powder, salt and vanilla extract. Stir until smooth. Pour over rhubarb and spread evenly with a spatula.
In another small bowl, combine powdered sugar and cornstarch. Sprinkle mixture evenly over top of rhubarb and batter mixture.
Pour water over top.
Bake for 1 hour.
Remove from oven, cool, then serve.
Wrap in saran wrap and store in fridge.