Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Grease a 9x9-inch baking dish with butter or non-stick spray.
Prepare the Rhubarb: Chop the rhubarb into 1-inch pieces and spread it evenly in the bottom of the prepared baking dish. If you prefer a sweeter pudding, sprinkle an additional tablespoon of sugar over the rhubarb.
Make the Batter: In a medium bowl, cream together the softened butter and 1 cup of sugar until light and fluffy. Beat in the egg and vanilla extract. In another bowl, whisk together the flour, baking powder, salt, and cornstarch. Gradually add the dry ingredients into the butter mixture, alternating with the milk, and mix until smooth.
Assemble the Pudding: Pour the batter over the rhubarb, making sure to cover it evenly. It may seem thick, but it will spread as it bakes.
Bake: Place the baking dish in the preheated oven and bake for 45–50 minutes, or until the top is golden brown and the pudding is bubbling. A toothpick inserted into the center should come out clean.
Cool and Serve: Let the pudding cool for about 10 minutes before serving. It’s best served warm, with whipped cream or vanilla ice cream on top.