1. Preheat the oven to 425° (225 c°). Coat a round pie tin with butter. Chop the rhubarb into 1/2 inch pieces. In a bowl, roll the rhubarb in a 1/2 cup of sugar and the potato flour until coated. Put the rhubarb mix in the pie tin.
2. In a separate bowl, add the flour and a tbs of the sugar. Break the butter into bits and add to the flour and sugar mixture, finely mix the butter into the dry ingredients, forming the ?crumbs?.
3. Add the crumb mix onto the rhubarb.
4. Bake in the middle of the oven for 30 mins, until golden brown. Serve with warm vanilla sauce.