Preheat the oven to 350 degrees F. Spray a 9 x 13 cake pan with nonstick spray.
Add berries to a large pot over medium high heat. Add in cornstarch, brown sugar, and cinnamon if desired. Heat to boiling. Boil 7 to 10 minutes, stirring constantly, until slightly thickened. Spoon into the prepared cake pan.
Evenly top with the DRY cake mix.
Covering the cake mix as much as possible, pour the soda over evenly. (Don't mix soda and cake mix so you get the crumbly topping instead of cake topping!)
Bake for 45-55 minutes or until the a toothpick inserted to the cake comes out clean.