Place a rack in the center of the oven. Preheat to 350 degrees. Lightly mist 12-cup Bundt pan with vegetable oil spray, then dust with flour. Shake out excess flour. Set pan aside.
Place cake mix, pudding mix, sour cream, water, oil and eggs in large mixing bowl. Blend with electric mixer on low speed for 1 minute. Stop and scrape down sides of bowl with a rubber spatula. Increase mixer speed to medium and beat 2 to 3 minutes more, scraping sides down again if needed. Batter should look thick and well combined. Fold in chocolate chips for cake, making sure they are well distributed throughout batter. Pour batter into prepared pan, smoothing out with rubber spatula. Place pan in oven.
Bake cake until it springs back when lightly pressed with finger and is just starting to pull away from sides of pan, 58 to 62 minutes. Run long, sharp knife around edge of cake and invert onto rack to cool completely, 20 minutes more.
Prepare icing by placing sugar, butter and milk in medium-size saucepan over medium-high heat. Stir until mixture comes to boil, 3 to 4 minutes. Still stirring, let mixture boil until sugar dissolves, 1 minute. Remove pan from heat. Stir in chocolate chips, and continue stirring until mixture is smooth and chocolate has melted.
Place cake on serving platter and pour warm icing over it. Let cake rest for 10 minutes before slicing, then slice and serve. (Store cake wrapped in aluminum foil or plastic wrap, or in cake saver, at room temperature for up to 1 week. Or freeze, wrapped in foil, for up to 6 months. Thaw cake overnight in refrigerator before serving.)