Line an 8x8-inch square baking pan with parchment paper, leaving an overhang on two opposite sides to create handles. Lightly spray the parchment with nonstick spray or brush with a bit of melted butter.

In a medium heavy-bottomed saucepan, combine the white chocolate chips, sweetened condensed milk, and butter. Set the pan over low heat and stir constantly with a silicone spatula until the mixture is completely smooth and glossy with no visible chips, about 4–6 minutes. Keep the heat low so the white chocolate does not overheat or seize.

Turn off the heat. Add the mashed banana, instant banana cream pudding mix, banana extract (if using), vanilla extract, and salt. Stir well until the pudding mix is fully dissolved and the mixture is smooth and slightly thickened.

Add the marshmallow creme to the warm mixture. Gently fold and swirl it in until there are no large pockets of marshmallow left. For a fully uniform fudge, fold it in completely; for a swirled look, leave a few streaks.

Quickly pour the banana cream fudge mixture into the prepared pan; it will thicken as it cools. Use an offset spatula or the back of a spoon to smooth the top into an even layer. If desired, sprinkle the surface with crushed vanilla wafers or graham crackers, chopped nuts, and lightly crushed banana chips, pressing gently so they adhere.

Cover the pan loosely with plastic wrap or foil and refrigerate for at least 3–4 hours, or until the fudge is firm enough to cut cleanly. In a warm kitchen or if you used a slightly larger banana, chill closer to 6 hours or overnight for the cleanest cuts.

Use the parchment overhang to lift the set fudge out of the pan and onto a cutting board. With a sharp knife, cut into 1-inch squares (or desired size). For neat edges, run the knife under hot water, wipe dry, and slice, repeating as needed. Drizzle with melted white or dark chocolate before serving if desired.