In a large bowl, combine cake mix and soda; beat on low speed for 30 seconds. Beat on medium for 2 minutes. Pour into a 13x9-in. baking pan coated with cooking spray. Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.
Meanwhile, in a large bowl, whisk 2-1/2 cups milk and pudding mix for 2 minutes. Let stand for 2 minutes or until soft-set. Fold in whipped topping; set aside. In a microwave, melt chocolate chips and remaining milk; stir until smooth. Coarsely crush five crackers; set aside. Cut cake into 1-in. cubes.
In a 3-qt. trifle bowl or glass bowl, layer half of the cake cubes, pudding, crushed crackers, chocolate mixture and marshmallow creme. Repeat layers. Refrigerate for at least 1 hour before serving. Just before serving, coarsely crush remaining crackers; sprinkle over the top. Garnish with grated chocolate if desired.