Line a square pan: Line a 9x9 square pan with parchment paper.
Boil: In a large saucepan, combine 3 cups sugar, 3/4 cup butter and 2/3 cup evaporated milk. Over medium heat, bring the mixture to a boil and cook until it comes together (about 5 minutes). It should reach 233°F with a candy thermometer. Stir occasionally.
Stir in chocolate: Remove from heat and stir in 2 cups chocolate chips until melted and smooth.
Add in the jar of marshmallow fluff, the 1 cup of walnuts and 1 teaspoon vanilla. Stir until all combined.
Pour into prepared pan and smooth out with a rubber spatula. You can optionally add some extra walnuts to the top. Refrigerate for 2 hours, until set.
Cut and serve: Score the top into 5x5 squares with a knife before cutting. Cut and serve.
Notes
Note: The original recipe calls for margarine, however we prefer salted butter. If you use unsalted butter, add in a pinch of salt. You can also use 1 bag (10 ounces) mini marshmallows instead of fluff.Storage: Store in a covered container for 1 week. You can also freeze up to 3 months.