Preheat the oven to 350℉.
Now, make the yellow cake mix according to the instructions and use a 9x13 baking pan. Once the cake is finished. Allow it to cool.
After the cake has cooled, poke holes using something circular like at the handle of a wooden spoon. Warm the caramel up slightly so it's easier to pour. Mix the condensed milk into the caramel. Then, pour on the top of the cake into the holes you just made. Some will run off to the sides but that's fine.
Place the cake in the fridge for a few hours so that the caramel and milk stiffens back up. When ready to serve, spread the cool whip across the top of the cake then add chopped up pieces of Twix.
Cut the cake into 12 square pieces then serve.