Preheat your oven to 325°F and grease a 9x13 inch baking dish (or similar size pan). In a large mixing bowl, combine the cake mix, room temperature eggs, oil, water, and milk, stirring until the batter is smooth and well combined. Room temperature eggs incorporate more easily into the batter, creating a better texture in the finished cake.
Pour the batter into your prepared baking dish and bake for 50-60 minutes, or until a toothpick inserted in the center comes out clean. The cake should be golden brown on top and spring back slightly when lightly touched.
Remove the cake from the oven and let it cool for about 20 minutes at room temperature—this gives it just enough firmness to hold its shape when you poke it. Using a fork or skewer, poke holes about halfway down into the cake, spacing them roughly an inch apart across the entire surface. I like to poke in a gentle pattern rather than pressing hard, which keeps the cake structure intact while still creating openings for the gelatin to seep into.
Dissolve the cranberry gelatin powder in 1 cup of boiling water, stirring for about 2 minutes until completely dissolved and the mixture is clear. Slowly pour the warm gelatin over the poked holes and across the entire cake surface, allowing it to seep into the holes and settle on top. The warm gelatin will absorb into the cake while cooling, infusing every layer with cranberry flavor.
Place the cake in the refrigerator to chill for at least 4 hours or overnight—this allows the gelatin to set completely and the flavors to meld together. While the cake chills, prepare the topping by mashing the cranberry sauce in a bowl until it reaches your desired consistency (I like to leave it slightly chunky for texture), then gently fold in the thawed whipped topping until well combined.
Remove the chilled cake from the refrigerator and spread the cranberry-whipped topping mixture evenly over the entire surface using a spatula or the back of a spoon. Garnish with fresh orange zest, which adds a bright citrus note that cuts through the sweetness and complements the cranberry flavors beautifully.