Place A Rack In The Center Of The Oven, And Preheat The Oven To 350°F.
Grease And Flour A 13″ × 9″ Baking Pan, And Shake Out The Excess Flour.
Set Aside.
Place The Chopped Rhubarb And ½ Cup Of The Sugar In A Small Glass Bowl.
Stir And Let The Mixture Rest While You Prepare The Rest Of The Cake.
Place The Butter And The Remaining 1½ Cups Sugar In A Large Mixing Bowl, And Beat With An Electric Mixer On Medium Speed Until Well Combined And Creamy, About 1 Minute.
Add The Eggs, One At A Time, Beating On Medium Speed Until Light And Fluffy.
Set Aside.
Sift The Flour, Baking Soda, Cinnamon, And Salt Into A Medium-Size Bowl.
Add This Flour Mixture To The Creamed Butter And Sugar Alternately With The Buttermilk, Beginning And Ending With The Flour.
Beat On Low Speed Until Combined.
Fold In The Rhubarb And Sugar Mixture And The Vanilla.
Pour The Batter Into The Prepared Pan, And Place The Pan In The Oven.
Bake The Cake Until It Is Golden Brown And A Toothpick Inserted In The Center Comes Out Clean, 38 To 42 Minutes.
Remove The Cake To A Rack And Let Cool In The Pan For 30 Minutes.
Dust The Top With The Confectioners’ Sugar, If Desired, Slice, And Serve From The Pan.