In a large mixing bowl, combine the vanilla pudding mix and sour cream until smooth.
Add 2 tablespoons of reserved pineapple juice to the pudding mixture and stir until slightly loosened.
Fold in the drained crushed pineapple and mix well.
Add the mini marshmallows and stir gently until evenly distributed.
Carefully fold in the whipped topping using a rubber spatula with gentle motions to maintain the airy texture.
If using optional add-ins like coconut or chopped nuts, fold them in now with just a few strokes.
Transfer the mixture to a serving bowl and smooth the top.
Cover with plastic wrap and refrigerate for at least 1 hour, though 2-3 hours is ideal for the flavors to develop.
Before serving, garnish with chopped pistachios or maraschino cherries if desired.
