In a saucepan, combine banana, chocolate, brown and granulated sugars, milk, salt and corn syrup. Stirring constantly, cook mixture over medium heat until the sugars dissolve. If sugar crystals form on the sides of the pan, wipe them off with a pastry brush dipped in water. Clip a candy thermometer to the side of the saucepan, and cook over medium heat, stirring the mixture occasionally to prevent sticking, until it reaches the soft-ball stage 236ºF). Remove the pan from the heat. Add the butter without stirring; cool the mixture until lukewarm (110ºF). Add vanilla extract; beat fudge until it loses its gloss and starts to thicken. Pour it into a buttered loaf pan. Sprinkle the fudge with chopped walnuts, if desired. Gently press walnuts into the fudge with a spoon. When fudge is cool and firm, cut the fudge into 32 pieces.