Preheat your oven to 350°F and lightly spray a 13x9-inch baking dish with vegetable oil spray.
In a large bowl, combine the cake mix, eggs, vegetable oil, and water. Beat the mixture on medium-high speed for 2-3 minutes until smooth and well combined.
Pour the batter into the prepared baking dish. Bake for 30 minutes, or as directed on the cake mix box. To check if it’s done, insert a toothpick into the center of the cake; if it comes out clean, the cake is ready. Remove from the oven and allow it to cool for 20-30 minutes.
Once the cake has cooled slightly, use a wooden skewer or fork to poke holes all over the surface. Pour the sweetened condensed milk evenly over the cake, tilting the pan to ensure it seeps into the holes. Drizzle the caramel sauce over the top. Let the cake cool to room temperature, then refrigerate for at least 2 hours or cover and chill overnight.
When you're ready to serve, spread the Cool Whip evenly over the cake. Sprinkle the crushed Butterfinger candy bars on top. Slice into squares and serve.
Notes
When poking holes in the cake, make sure they go all the way to the bottom and are evenly spaced. This allows the condensed milk and caramel to soak into every bite.
Allowing the cake to chill for several hours, or even overnight, helps the flavors meld together and makes it easier to slice.
Keep the cake refrigerated until you’re ready to serve it. The cold helps the cake hold its shape better, especially after adding the Cool Whip.