Preheat the oven to 375F (non-convection).

Set some water to boil for adding later in step 3.

Cover the bottom of an 8-inch square or round pan with the diced rhubarb.

Make the batter: In a medium bowl, mix together the sugar, baking powder, flour and salt. Add the butter, vanilla and milk. Mix together with a wooden spoon, using the spoon to press any larger pieces of butter against the bottom of the bowl to break them up. The batter doesn't need to be completely smooth. A few butter bits are fine.

Make the topping: In a small bowl, whisk together the sugar and cornstarch. Sprinkle over the batter in the baking pan. Place a spoon on top with the curved side up. Pour boiling water over the curved part of the spoon and allow it to spread overtop of the sugar mixture.

Place the baking pan on top of a baking sheet to catch any bubbleovers. Bake at 375°F for 45-60 minutes or until the cake layer is set and the top is golden.

Remove from the oven and allow to stand a few minutes before enjoying warm plain or with a scoop of vanilla ice cream.