Prepare the vanilla pudding with the cold milk according to package directions.
Stir in the vanilla extract.
Fold in whipped heavy cream if using.
Refrigerate the filling for 15–20 minutes until thick.
Separate the Hawaiian rolls.
Cut a small opening into the side of each roll without slicing all the way through.
Optional: Warm the rolls in a 325°F (165°C) oven for approximately 5 minutes, then cool.
Transfer the vanilla filling to a piping bag.
Pipe the filling into each roll until comfortably filled.
Place the chopped chocolate in a heat-safe bowl.
Heat the heavy cream until steaming but not boiling.
Pour the hot cream over the chocolate and let it sit for 2 minutes.
Stir gently until smooth.
Add butter if using.
Dip the top of each filled roll into the chocolate ganache.
Place on a tray and allow the ganache to set for 15–20 minutes.
Refrigerate until ready to serve.