Place a rack in the center of the oven and preheat the oven to 350 degrees. Lightly mist a 10-inch tube pan with vegetable oil spray and dust it with flour. Shake out the excess flour. Set the tube pan aside.
Place the cake mix, flour, butter, granulated sugar, chocolate syrup, 1 egg, and 1/2 cup of water in a large mixing bowl. Beat with an electric mixer on low speed for 30 seconds.
Add the remaining eggs one at a time, beating on low speed until each is incorporated, stopping in between to scrape down the side of the bowl with a rubber spatula.
When all of the eggs have been added, increase the mixer speed to medium and beat the batter 1 minute longer, scraping down the side of the bowl again if needed. The batter should look smooth and thick. Transfer the batter to the prepared tube pan, smoothing the top with a rubber spatula.
Bake the cake until the top springs back when lightly pressed with a finger, 50 to 55 minutes. Transfer the tube pan to a wire rack and let the cake cool for about 20 minutes.
Run a long, sharp knife around the edges of the cake and invert it onto a wire rack, then invert it again onto another wire rack so that the cake is right side up. Let the cake cool completely, about 20 minutes. Dust the cake with confectioner's sugar or top it with a chocolate glaze.