Preheat oven to 350°

1. In a medium saucepan whisk together the sugar, water and cornstarch.
   
   
2. Bring to a boil, reduce the heat and simmer until the mixture is very thick, about 5–7 minutes.
   
   
3. Remove from the heat and stir in the rhubarb and vanilla.
   
   
4. In a medium bowl whisk together the flour, oats, cinnamon and brown sugar.
   
   
5. Stir in the melted butter and vanilla and mix until moist and well combined.
   
   
6. Press half the oat mixture into the bottom of an 8x8-inch pan.
   
   
7. Spread the rhubarb mixture on top and sprinkle with the remaining oatmeal mixture.
   
   
8. Bake for 1 hour.
   
   
9. Serve with vanilla ice cream.