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.
Video
Notes
*For thicker fudge, pour the mixture into an 8x8-inch or 9x9-inch parchment paper-lined pan.