Heat oven to 350. Spray a 9" x 13" baking dish or pan w/ cooking spray. In bowl, prepare cake mix as directed, but, instead of using the called-for water, use the 1-1/3 cups coca-cola soda.
Add dry pudding mix. Mix as directed. Pour batter into pan. Bake 35-39 mins, or until a toothpick inserted in center comes out clean.
Meanwhile make frosting; In medium saucepan over medium heat, melt butter. Add in cocoa powder and light cream. Stirring constantly. Whisk in powdered sugar until smooth. Take the pan off the heat and stir in vanilla. Pour frosting evenly over cake and serve.