Preheat oven to 350 degrees. Melt the butter in a 9x9 pan, add the diced rhubarb and sugar mixed together and spread over the butter, covering the bottom of the pan. If you are using frozen rhubarb put it in the oven to thaw while you mix up the batter.
Sift the sugar, flour, baking powder and salt; add the soft shortening and cut it into the flour mixture.
Beat the egg in a cup, stir in the vanilla and fill the cup with milk, beating all together.
Pour the liquid over the flour mixture and blend until smooth.
Spread the batter over the rhubarb evenly.
Bake at 350 degrees for about 30-40 minutes until the cake tests done.
Serve warm with a scoop of vanilla icecream. Don't turn the whole cake upside down - simply cut it in squares and flip each piece rhubarb side up when serving.
Variation: If you want to make a gingerbread version, add 1/2 tsp each of cinnamon, nutmeg, ginger and 1/2 cup molasses instead of half the sugar.