Preheat oven to 350°F and grease a 9×13 inch cake pan.
In a large bowl, combine the cake mix, cold water, vegetable oil, and eggs. Beat on low, then on medium for 2 minutes until smooth.
Pour batter into the prepared pan and bake for 23 minutes, or until a toothpick inserted in the center comes out clean.
Allow cake to cool on a wire rack for about 20 minutes.
In a small bowl, dissolve the orange Jello in boiling water, stirring with a whisk until completely dissolved.
Use a chopstick or wooden spoon handle to poke holes about 1 inch apart across the top of the cooled cake.
Slowly pour the orange Jello evenly over the cake so it seeps into the holes. Let the cake absorb for about 30 minutes.
In a large bowl, whisk together the vanilla instant pudding and cold milk until smooth.
Fold in the Cool Whip and vanilla extract until the frosting is smooth and well combined.
Spread the pudding–Cool Whip frosting evenly over the top of the cake with an offset spatula.
Refrigerate the cake for about 1 hour to set before slicing and serving.
Slice into 12 pieces and serve chilled, garnished with mandarin oranges if desired.