Preheat the oven to 350 degrees F (175 degrees C). Grease an 8 inch square baking dish.
In a medium bowl, stir together the white sugar, flour and baking powder. Stir in the milk until blended, then stir in the rhubarb. Pour into the prepared baking dish and spread into an even layer. In a separate bowl, stir together the brown sugar, butter and boiling water until butter has melted. Pour over the cake in the pan.
Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, 45 to 50 minutes.
