Preheat oven to 350° F. Mix butter and sugar together in a large bowl. Add the eggs and stir.

Add the flour, baking powder, milk, vanilla, and salt. Mix until it gets to be a smooth, thick batter, and pour into a buttered 9 x 13 pan.

In a large bowl, mix the rhubarb pieces with the gelatin powder and 1/2 cup sugar, and pour over the batter.

To prepare the topping: Crumble the flour, sugar, and butter together in a small bowl, and sprinkle over the rhubarb mixture. Bake for 40 to 60 minutes, until toothpick comes out dry. Serve warm or cool, topped with whipped cream.