Line your 8-inch square baking pan with aluminum foil or parchment paper. Leave a little extra hanging over the sides to help you lift the fudge out later.

In your medium saucepan, combine the white sugar, evaporated milk, butter, and salt over medium heat. Stir the mixture constantly to prevent the sugar from sticking to the bottom.

Bring the milk and sugar mixture to a full, bubbling boil. Let it boil for exactly four to five minutes while stirring it very gently with your spatula.

Remove the saucepan from the heat immediately after the boiling time is up. Stir in the miniature marshmallows until they are mostly melted and look very fluffy.

Add the semi-sweet chocolate chips to the warm mixture and stir quickly. The heat from the boiled milk will melt the chocolate chips into a thick sauce.

Stir in the pure vanilla extract and the optional chopped nuts at this stage. The vanilla helps balance the intense sweetness of the chocolate and the marshmallows.

Continue to stir the fudge mixture vigorously for about one to two minutes. You want the fudge to look glossy, thick, and completely smooth without any lumps.

Pour the warm chocolate mixture into your prepared baking pan. Use your spatula to smooth the top so it is even and looks very neat.

Place the pan in the refrigerator and let it chill for at least two hours. This allows the fudge to become firm enough to slice into perfect squares.

Lift the fudge out of the pan using the foil edges. Use a sharp knife to cut it into small squares and serve it to your guests.