Preheat oven to 350℉ (175℃).
In a large bowl, whisk flour, sugar, baking soda, salt, and cinnamon (if using).
Pour the Coca-Cola into a small saucepan and cook for about 15 minutes until it reduces to 1½ cups.
Add butter and cocoa powder. Bring to barely a boil, stirring occasionally.
Stir the Coca-Cola mixture into the flour mixture until just combined.
In a small bowl, whisk the eggs, buttermilk, and vanilla, then stir them into the Coca-Cola batter until just combined.
Transfer the batter to a greased 9"x13" baking pan and bake for 25-30 minutes. A toothpick inserted in the center should come out clean.