Preheat your oven to 350°F (175°C) and grease a 9-inch square baking dish.

Chop fresh rhubarb into bite-sized pieces and spread them evenly across the bottom of the dish. Sprinkle granulated sugar and drizzle lemon juice over the rhubarb.

In a mixing bowl, whisk together flour, baking powder, and salt. Add in milk, melted butter, and vanilla extract, stirring until smooth.

Gently pour the cake batter over the rhubarb layer without stirring, spreading it evenly with a spatula.

Combine brown sugar and cornstarch in a bowl, then sprinkle over the batter.

Pour boiling water over the entire surface without stirring, covering the sugar layer.

Bake for 40-45 minutes until golden brown and bubbly around the edges.

Let the pudding cool slightly on a wire rack for about 10 minutes before serving.