Mash bananas until mostly smooth; small lumps are fine.
Add mashed bananas, butter, sugar, heavy cream, and salt to a medium saucepan.
Heat over medium, stirring constantly until everything blends.
Once bubbling, maintain a gentle boil.
Cook 8–10 minutes, stirring often.
If using a thermometer: 234–238°F (soft-ball stage).
Remove from heat; stir in white chocolate until melted and smooth.
Add marshmallow crème and vanilla; mix until silky and uniform.
Line an 8×8-inch pan with parchment.
Pour in fudge, spread evenly, and sprinkle toppings if using.
Refrigerate 3–4 hours until firm.
Lift out with parchment and cut into squares using a warm knife.