Step 1

Line a 9 x 13- inch baking pan with parchment, leaving sides overhanging the pan.

Step 2

In a medium saucepan, combine the sugar and milk and bring to a boil over medium high heat. Cook at a rolling boil for 7 minutes. The mixture will bubble up, so be sure the pan is large enough to accommodate this and watch carefully, stirring often.

Step 3

In a large bowl, stir the marshmallow fluff, butter, and vanilla together until mixed, then stir in the chocolate chips. Pour the boiling sugar mixture into the bowl and stir with a wooden spoon or wide spatula until all white streaks disappear. Stir in walnuts.

Step 4

Pour fudge into the prepared pan and spread evenly. Chill 30 minutes before cutting into squares to serve.