Preheat oven to 350 degrees F. Lightly grease a 9x13" cake pan with cooking spray and set aside.
Combine the cake mix, 1 ¼ cup of cold root beer soda, canola oil and eggs in a large bowl. Beat on low speed for 30 seconds, then increase speed to medium for an additional 2 minutes. Batter should be light and fluffy.
Pour cake batter into the prepared pan and bake for 30 to 35 minutes, until toothpick inserted in center comes out clean. Let the cake cool completely.
For The Frosting
Combine 1 cup of the cold root beer soda along with 2 packets of the Dream Whip topping mix in a large bowl and beat on low speed for 30 seconds. Increase the speed to medium and continue beating until soft peaks form. Spread the topping over the cooled cake. Slice and enjoy!
Store cake covered in the fridge for up to 3 days.