Preheat your oven to 350°F (175°C) and generously grease a 9x13 inch baking pan with butter or non-stick spray.

In a large mixing bowl, cream together the softened butter and 1 1/4 cups of sugar until the mixture is light and fluffy.

Add the egg and vanilla extract to the butter mixture, beating well until fully incorporated.

In a medium bowl, whisk together the flour, baking soda, and salt to ensure even distribution of the leavening agent.

Alternately add the dry ingredients and the sour cream to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.

Gently fold in the chopped rhubarb using a spatula, being careful not to overmix the batter.

Spread the batter evenly into the prepared baking pan. In a small bowl, combine the remaining 1/4 cup sugar and cinnamon, then sprinkle this mixture over the top of the batter.

Bake for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool slightly before serving.