Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, cream together the granulated sugar and softened butter until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and sour cream until combined.

Gradually add the dry ingredients to the wet mixture, mixing just until combined. Gently fold in the chopped rhubarb.

Pour half of the batter into the prepared baking dish and spread it evenly. In a small bowl, mix together the brown sugar and cinnamon, then sprinkle half of this mixture over the batter. Pour the remaining batter on top and spread it evenly. Finally, sprinkle the rest of the brown sugar and cinnamon mixture over the top.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for about 10 minutes before slicing and serving.