1. Preheat oven to 350 degree F and line two 12-cup muffin pans with paper liners.
2. Blend cake mix, mashed bananas, water, oil, eggs, and cinnamon in a large bowl with an electric mixer set on low speed for 30 seconds until just combined. Stop mixer and scrape down the sides of the bowl with a rubber spatula. Increase mixer speed to medium and beat 2 minutes more, scraping down sides again, if needed (the batter should look thick and well combined). Spoon or scoop batter into each cup, filling it three-quarters of the way.
3. Bake in middle of oven until cupcakes are golden and spring back when lightly pressed, 18 to 20 minutes. Let cupcakes cool in pans on racks, 5 minutes. Remove cupcakes from pans and place on racks to cool for 15 minutes before frosting.
4. Place a heaping tablespoon of frosting on each cupcake and swirl to spread it smoothly with a short metal spatula or spoon, covering the top completely. Decorate as shown in photograph (above).