Preheat the oven to 350 degrees F (175 degrees C). Grease two 9x5-inch loaf pans and line the bottoms with parchment paper.

Sift flour, cocoa powder, cinnamon, salt, baking powder, and baking soda together into a bowl. Whisk buttermilk, water, and vanilla extract together in a separate bowl.

Combine butter and sugar in a third, large bowl; beat with an electric mixer until light and creamy, about 5 minutes. Add eggs, one at a time, beating after each addition until egg is completely incorporated. Add flour mixture in 3 batches, alternating with buttermilk mixture, beating batter briefly after each addition until just blended. Divide batter evenly between the prepared loaf pans.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 45 to 50 minutes. Cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Invert carefully onto a serving plate or cooling rack. Let cool completely.