Once it is cooled, start the cake. Line a 9x13 pan with parchment paper, set aside. If you need help getting the parchment paper to stick to the pan, spray the pan lightly with cooking spray.
In a large mixing bowl, combine 1/2 of the gallon of ice cream, 1 tablespoon root beer extract, 1/4 cup root beer, and Cool Whip; stir until combined. Then add in the remaining ice cream, stir until the two ice creams start to marble together, make sure not to completely stir them together, you want to see a slight swirl.
Add the ice cream to the prepared pan. Top immediately with 2/3 cup of the cooled Root Beer Caramel.
Using a knife, swirl the caramel into the cake.
Set in the freezer until set 2-3 hours.
When ready to serve prepare whipping cream. Whisk together with an electric mixer or beater heavy cream and sugar. Beat until it starts to form soft peaks. Store in the refrigerator while you cut the cake.
Cut the cake into 12-square pieces and serve immediately topped with whipped cream.
Option: If you want, once you top with ice cream pour about ¼ cup of root beer into the bowl, then serve, this adds the float element.
If you want the round pieces use a biscuit cutter, like pictured. With a biscuit cutter you will get 8-9 pieces. With the left over ice cream just make a root beer float. If using the biscuit cutter you may need to let the cake reset by setting it in the freezer for an 1 hour if it starts to melt.