Preheat the oven to 350 degrees and grease the baking dish with butter.
In a stand mixer fitted with the paddle attachment, combine the sourdough starter, honey, softened butter, eggs, and vanilla. Mix on medium speed until well combined.
Turn the mixer off and add the flour, baking soda, salt, and ground cinnamon. Turn the mixer on low and mix until well combined.
Turn the mixer off, add the chopped rhubarb, and stir in by hand using a large spoon or spatula.
Spread the cake batter evenly into the baking dish.
Bake for 45 minutes, or until lightly browned on top and a knife inserted into the center comes out clean.
Allow the cake to cool completely before frosting, if desired.
Notes
Store leftover cake in an airtight container in the fridge for up to a week.