Preheat your oven to 350°F (175°C) and grease a baking pan.
In a mixing bowl, combine the cake mix, milk, melted butter, eggs, and vanilla extract. Mix until smooth.
Fold in the chopped rhubarb and pour the batter into the prepared baking pan.
In another bowl, mix together the granulated sugar, cornstarch, softened butter, brown sugar, and cinnamon to create the filling. Sprinkle it over the batter.
Bake in the oven for 35 minutes or until a toothpick comes out clean.
For the glaze, whisk together powdered sugar, milk, and vanilla extract in a small bowl. Drizzle it over the cooled cake before serving.
Notes
Store leftovers in an airtight container for up to three days.