Preheat the oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In another bowl, mix the sour cream, softened butter, eggs, and vanilla extract until smooth.

Combine the wet ingredients with the dry ingredients and mix until just combined.

In a separate bowl, combine the chopped rhubarb, sugar, and cornstarch. Toss to coat the rhubarb evenly.

Spread half of the cake batter into the prepared pan. Layer the rhubarb mixture on top.

Spread the remaining cake batter over the rhubarb filling.

In a small bowl, mix together the brown sugar, flour, melted butter, and cinnamon for the topping. Sprinkle it over the cake.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Slice and serve warm or at room temperature.