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. *You can also use a deep-dish pie pan or a ceramic baking dish of similar size.
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. Just avoid larger pieces. Spoon the batter over the rhubarb in the baking dish and spread to cover the rhubarb in a thin layer.
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. Frozen rhubarb will need extra baking time and be sure to test the cake in the centre of the pan with a cake tester to ensure that it is set. If the cake is at risk of over-browning before the cake is set, loosely lay a sheet of aluminum foil overtop for the last part of baking.
Remove from the oven and allow to stand a few minutes before enjoying warm plain or with a scoop of vanilla ice cream.
Refrigerate leftovers for 2-3 days. Reheat in the microwave or cover with foil and pop into a 350F oven until warmed about 10 minutes. Leftovers will also freeze well up to 3 months.
Notes
Note 1: Adjust the amount of sugar for the topping to your preference for sweetness. Use the lesser amount for a more tart rhubarb flavour or the higher amount for more sweet rhubarb flavour. Be sure to read the notes above this Recipe Card, where I share some valuable tips, options, substitutions and variations for this recipe! You'll also find step-by-step photos as well as a recipe video that you might find helpful.