In a large saucepan, add the sugar, salt, butter, cream, and marshmallows. Cook over medium heat until the marshmallows and butter begin to melt (about 5 minutes).
Once the marshmallows have melted, increase the heat and bring the mixture to a boil. Bring to a boil without stirring. After about 3 minutes, the mixture will begin to brown and bubble furiously. As soon as that happens, remove the saucepan from the heat.
Mix in the chocolate chips and vanilla extract until the chocolate has melted and the fudge mixture is smooth.
Line a 9x13-inch pan with parchment paper and spray with cooking spray. Pour the mixture into the pan.
If adding toppings, be sure to do so immediately, before the fudge begins to harden. After decorating, let the fudge sit at room temperature for 3 hours before slicing.
Once the fudge is firm, cut it into squares.