Grease an 8×8-inch baking pan or line it with parchment paper.
Measure all ingredients ahead of time for smooth preparation.
In a medium saucepan, combine sugar, evaporated milk, butter, and salt.
Place saucepan over medium heat and stir constantly until the mixture comes to a gentle boil.
Simmer for exactly 5 minutes, stirring continuously to prevent sticking or burning.
Remove the pan from heat and immediately stir in semi-sweet chocolate chips until melted and smooth.
Add mini marshmallows, vanilla extract, and chopped pecans if using; stir until marshmallows melt and the mixture is thick and glossy.
Pour the fudge into the prepared pan and spread evenly with a spatula.
Allow the fudge to cool at room temperature for 1 to 2 hours until fully set.
Once firm, cut into 16 squares and serve or store in an airtight container.