Line 9-inch square pan with aluminum foil or parchment paper, crisscrossing to make a sling.
Combine the 3 cups granulated sugar, ¾ cup unsalted butter and ⅔ cup evaporated milk in a heavy-bottom saucepan. Bring to a rolling boil over medium heat, stirring constantly. Simmer and stir for 5-10 minutes or until it registers at 234°F on a candy thermometer. Remove from the heat.
Add the 12 ounces semisweet chocolate chips and 1 cup marshmallow fluff to the hot mixture; stir until melted. Add the chopped 1 cup chopped walnuts and 1 teaspoon vanilla extract.
Pour the hot fudge into the prepared pan; spreading evenly. Cool completely. Use foil handles to lift fudge from pan before cutting into 1-inch squares.
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Notes
Storage: Store fudge in an airtight container or wrapped in plastic wrap at room temperature for 2-3 weeks. Be sure to keep the fudge in a dry, cool place and avoid refrigerating because it will dry out.Freezer: This classic recipe can also be frozen. Freeze it for up to three months in a freezer bag, but keep in mind it will be a little drier when thawed. Also, place wax paper in between fudge pieces to prevent them from sticking.