Preheat and Prep: Preheat your oven to 350°F (175°C). Grease and lightly flour two 8-inch or 9-inch round cake pans.

Mix Batter: In a large bowl, combine the yellow cake mix, vegetable oil, eggs, mandarin oranges, and ½ cup of the reserved orange juice. Beat with an electric mixer on medium speed for 2 minutes until the oranges break into small pieces and the batter is well combined.

Bake: Divide the batter evenly between the prepared pans. Bake for 22 to 28 minutes, or until a toothpick inserted into the center comes out clean.

Cool: Let the cakes cool in the pans for 10 minutes, then turn them out onto wire racks to cool completely to room temperature.

Make Frosting: In a large bowl, mix the undrained crushed pineapple and the dry instant vanilla pudding mix until fully combined and thickened. Gently fold in the thawed whipped topping until smooth.

Assemble: Place one cooled cake layer on a serving plate and spread an even layer of frosting over the top. Add the second cake layer and use the remaining frosting to cover the top and sides of the entire cake.

Chill: Refrigerate the assembled cake for at least 4 hours (overnight is highly recommended) before slicing and serving.