Preheat oven to 325F. Line a 9x13-inch pan with aluminum foil for easier cleanup and spray with cooking spray; set aside.

To a large bowl, add the cake mix, eggs, sour cream, oil, and beat with a handheld electric mixer on high speed until mixed and combined, about 2 to 3 minutes; the batter is thick.

Transfer half the batter to the prepared pan; set remaining batter and pan aside. Tip - Make sure not to add more than half the batter or you will have a hard time with coverage in step 5.

To a medium bowl, add the brown sugar, cinnamon, stir to combine, and evenly sprinkle the cinnamon-sugar over the batter in the pan.

Evenly top with the remaining batter, lightly spreading it with a spatula to completely cover the brown sugar (it doesn't matter if it's not perfect).

Bake for about 35 minutes or until done and a toothpick comes out clean or with a few moist crumbs.

Poke the hot cake all over with a fork to create holes. I probably poked the cake in about 100 places, the more holes the better distributed the milk mixture will be (next step); set cake aside.

To a medium bowl, add the confectioners' sugar, milk, and whisk to combine until smooth and free from lumps; mixture will be thin, this is ok.

Evenly and slowly drizzle the milk mixture over the cake, cover, refrigerate, and allow cake to cool for about 2 hours (or overnight).

Evenly add the whipped topping over the surface of the cake and smooth it lightly with a spatula or knife before serving.