Drain pineapple. If pineapple is not well drained, it will make the filling runny.
Mix and bake cake according to package instructions. Pour 1/2 of the batter into the prepared pan. Discard or find another use for the remaining half of the batter.
Beat softened cream cheese until fluffy. Slowly add milk, combining thoroughly. Add instant pudding and beat until thick. Pudding mixture will thicken when refrigerated. When cake is cool and pudding mixture is thickened you can assemble the cake. Spread pudding mixture on top of the cooled cake. Sprinkle drained pineapple over pudding. Carefully spread thawed frozen whipped topping over all. Sprinkle chopped walnuts over all. Refrigerate. Enjoy!