Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a large bowl, cream together the butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir in the sour cream and vanilla.

In a separate bowl, whisk together flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture until just combined.

Fold in the chopped rhubarb.

Pour the batter into the prepared cake pan.

In a small bowl, combine cinnamon and brown sugar for the topping, then sprinkle it over the batter.

Bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean.

Allow to cool before serving.