Preheat the oven to 350°F (175°C).
Combine rhubarb, sugar, cornstarch, and water in a saucepan.
Cook over medium heat until the mixture thickens and bubbles, about 5-7 minutes.
Whisk together 1 cup all-purpose flour, 1 cup granulated sugar, 1 tsp baking powder, and 1/2 tsp salt in a mixing bowl.
Stir in 1/2 cup milk, 1/4 cup melted unsalted butter, and 1 tsp vanilla extract until just combined.
Pour the cake batter into a greased 9-inch baking dish.
Spoon the rhubarb mixture over the batter without stirring it together.
Bake in the preheated oven for 45 minutes or until the top is golden brown and a toothpick inserted comes out clean.