In a medium saucepan, combine the sugar, evaporated milk, butter, and salt. Place over medium heat and bring the mixture to a boil, stirring constantly to prevent burning.
Once the mixture has boiled, add the chocolate chips. Continue stirring until the chocolate is completely melted and the mixture is smooth.
Remove the saucepan from the heat. Quickly add the marshmallows, vanilla extract, and nuts (if using). Stir vigorously until the marshmallows are fully melted and everything is well combined.
Pour the fudge mixture into a greased 8-inch square pan. Smooth the top with a spatula.
Allow the fudge to cool at room temperature until set. Once cooled, cut into squares.
