In an 8 x 8 inch greased baking dish, arrange fresh diced rhubarb; sprinkle with cornstarch and stir until evenly coated.
For the cake, cream softened butter and sugar.
To the butter and sugar mixture, alternately add dry ingredients (flour, baking powder and salt) and milk and vanilla; beat until smooth.
Spread cake mixture evenly over rhubarb so that the batter completely is coated from edge to edge.
Combine sugar, cornstarch and salt; sprinkle entire mixture evenly over cake batter.
Pour boiling water over the sugar mixture until it is all moistened, approximately 1/2 cup.
Bake in a preheated 375 degree oven for 45-55 minutes or until golden brown and bubbly.