Preheat oven to 375 F (190 C). Prepare a 9 inch square baking pan
Cover the bottom of the baking pan with rhubarb and set aside.
Mix 3/4 cup sugar with flour, baking powder, salt, vanilla, butter and milk. Beat together until smooth. Pour over the rhubarb and smooth out.
Mix 1/2 cup sugar and cornstarch together and sprinkle over the batter, then pour the boiling water over the top. This is delicate; start pouring around the sides then into the centre so you do not have an indent in the middle of your cake from the water.
Bake for 45/50 minutes or until a toothpick comes out clean. The top will have a glaze over it.
You can use any fruit in this recipe. However you may wish to reduce the sugar in the glaze to compensate for the sweetness of the fruit.