Preheat your oven to 350°F (175°C). In a mixing bowl, combine the white cake mix, egg whites, water, and oil as per package instructions. Beat the mixture on medium speed for about 2 minutes until smooth and well blended. Pour the batter into a greased 9x13-inch baking pan and bake for 25–35 minutes, or until a toothpick inserted comes out clean. Let cool completely.

Once the cake has cooled, poke holes in the cake using a wooden spoon handle, ensuring the holes are evenly spaced about 1 inch apart.

In a mixing bowl, dissolve the orange gelatin in 1 cup of boiling water until fully dissolved. Add ½ cup of cold water and allow the mixture to thicken slightly before pouring over the poked cake. Let it soak for about 15 minutes.

Spread a thin layer of orange marmalade over the top of the cake, ensuring to reach the edges. Allow it to set for 5 minutes before frosting.

In a mixing bowl, whisk together the instant vanilla pudding mix and 2 cups of milk until thickened. Fold in the thawed whipped topping. Spread the frosting over the cake evenly.

Cover with plastic wrap and refrigerate for at least 4 hours or overnight for the best flavor and texture. Garnish with mandarin orange slices before serving.