Preheat your oven to 325 degrees Fahrenheit. While it’s heating up, line a 9x13-inch pan with aluminum foil for easier cleanup, then spray it with cooking spray. This will keep your cake from sticking.
In a large bowl, combine the cake mix, eggs, sour cream, and oil. Use a handheld electric mixer on high speed for 2 to 3 minutes until everything is well mixed and the batter is smooth. Look for a thick consistency.
Transfer half of the batter to the prepared pan and set the other half aside. Be careful not to add too much batter, as we’ll be layering it later.
In a medium bowl, mix the brown sugar and cinnamon together until well combined. This is your cinnamon-sugar filling. Evenly sprinkle this mixture over the batter in the pan.
Now, take the remaining batter and evenly top the cinnamon-sugar layer, using a spatula to spread it out. It doesn’t have to be perfect; just cover the sugar.
Place the pan in the oven and bake for about 35 minutes. To check for doneness, insert a toothpick into the center of the cake. It should come out clean or with a few moist crumbs.
Once baked, remove the cake from the oven and poke it all over with a fork. I recommend about 100 holes—you want the milk mixture to soak in well, so the more holes, the better.
In a medium bowl, whisk together the confectioners' sugar and milk until smooth. The mixture should be thin, but that’s perfectly fine.
Slowly drizzle the milk mixture evenly over the poked cake, allowing it to seep into the holes. Then, cover the cake and refrigerate for at least 2 hours, or overnight if you can wait.
Finally, take the whipped topping and spread it evenly over the cooled cake. You can use a spatula or a knife to smooth it out nicely before serving. Enjoy the deliciousness!