In a large saucepan over medium heat, mix the sugar, butter, and evaporated milk.
Bring the mixture to a boil. Leave on heat until the mixture registers 234°F on a candy thermometer (about 5 minutes).
Remove from heat and stir in chocolate chips until melted and thoroughly combined.
Add the mini-marshmallows, walnuts, and vanilla extract. Stir to incorporate.
Transfer the fudge to the prepared pan. Allow the fudge to set at room temperature (about 4 hours). To speed up the setting process, you can let it chill in the refrigerator to set.
Serve at room temperature.
Notes
This fudge is best enjoyed at room temperature. You can store it in an airtight container for up to a week.