Preheat your oven to 350°F (175°C). Grease and flour 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 dry ingredients to the butter mixture, alternating with the sour cream. Begin and end with the flour mixture, mixing just until combined.

Fold in the chopped rhubarb gently until evenly distributed.

In a small bowl, mix together the brown sugar and cinnamon.

Pour half of the batter into the prepared baking pan and sprinkle half of the brown sugar mixture over it. Spread the remaining batter on top and finish with the rest of the brown sugar mixture.

Bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for about 10 minutes before slicing and serving.