Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the flour mixture to the butter mixture, alternating with the sour cream, beginning and ending with the flour. Mix until just combined.
Fold in the chopped rhubarb gently until evenly distributed.
Pour the batter into the prepared baking pan and spread it evenly.
In a small bowl, combine the brown sugar and cinnamon. Sprinkle this mixture evenly over the top of the batter.
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 15 minutes before slicing and serving.