Prepare the pan: Line an 8-inch (20 cm) square pan with parchment paper, leaving enough overhang on two sides to lift the chilled fudge from the pan.

Mash the banana: Measure ½ cup (120 g) finely mashed ripe banana. Break up any large pieces so the mixture is as smooth as possible.

Cook the banana and butter: Melt 2 tablespoons (28 g) unsalted butter in a medium heavy-bottomed saucepan over medium-low heat. Add the mashed banana and cook for 2 to 3 minutes, stirring frequently. It should become slightly thicker and more fragrant without browning.

Add the condensed milk: Stir in the 14-ounce (396 g) can of sweetened condensed milk. Continue cooking over low heat for about 2 minutes, stirring constantly, until the mixture is hot and uniform. Do not let it boil vigorously.

Melt in the white chocolate: Reduce the heat to its lowest setting and add 3 cups (510 g) white chocolate chips. Stir continuously until the chocolate is completely melted and the fudge looks thick, smooth, and glossy, about 2 to 3 minutes. If the chocolate seems reluctant to melt, remove the pan from the heat and keep stirring rather than increasing the temperature.

Add the flavorings: Remove the saucepan from the heat. Stir in 1 teaspoon (5 ml) vanilla extract and ¼ teaspoon fine salt until evenly incorporated.

Fill the pan: Immediately scrape the warm fudge into the prepared pan. Spread it into an even layer, smoothing the top with the spatula.

Add the optional walnuts: Sprinkle ½ cup (55 g) toasted, finely chopped walnuts evenly over the surface. Press them gently into the fudge so they adhere.

Chill until firm: Let the pan stand for about 20 minutes to stop steaming, then cover and refrigerate for at least 3 hours 40 minutes. The center should feel firm rather than soft or fluid when gently pressed.

Cut into squares: Lift the chilled fudge from the pan using the parchment overhang. For 25 pieces, cut it into a 5-by-5 grid with a sharp knife, wiping the blade between cuts for cleaner edges.