Preheat oven to 350 degrees and grease a 9×13 baking dish.
In a large bowl, mix together the cake mix, pudding mix, oil, eggs, and pineapple with juices. Beat until well combined.
Transfer batter to the prepared baking dish and bake for 30-35 minutes or until an inserted tooth pick comes out clean. Allow cake to cool.
To make the frosting: Mix together the second pudding mix and milk until the mixture thickens. Fold in the cool whip until fully combined.
Frost cake and chill for 2 hours before serving. Top with chopped pistachios if desired.
