Step 1

In the bowl of your stand mixer fitted with the whisk attachment, whip the egg whites until they form soft peaks. Set the bowl aside briefly.

Step 2

In a medium saucepan, place the sugar, water, cream of tartar and salt, and whisk together.

Step 3

Cook the mixture over medium-high heat until it reaches the softball stage (between 235°F and 240°F on an instant read candy thermometer). Be sure not to cook the sugar on high heat, or the sugar and burn.

Step 4

Remove the cooked sugar from the heat, and pour carefully down the side of the stand mixer bowl, with the mixer on low speed (making sure the sugar mixture doesn’t hit the whisk).

Step 5

Add the vanilla extract, and increase the mixer to high speed. Beat until stiff peaks form (5 to 7 minutes).

Step 6

Line an 8-inch or 9-inch square pan with aluminum foil, allowing some foil to overhang the pan. Smooth out any wrinkles. Grease the foil completely (or use nonstick aluminum foil), and set the pan aside.

Step 7

In a clean, medium saucepan, place the butter, sugar, salt, and evaporated milk. Place the saucepan over medium to medium-high heat, and bring the mixture to a boil.

Step 8

Cook, stirring occasionally, until it reaches 235°F (about 6 minutes). If your mixture is taking a long time to reach the proper temperature, you can try covering the pan loosely with a cover, so less heat is escaping during cooking.

Step 9

Remove the pan from the heat. Working quickly, add the chopped chocolate and mix until the chocolate is melted. Add the vanilla, then the 7 ounces of marshmallow fluff. Using a silicone or other nonstick spatula, turn the mixture over gently until completely combined.

Step 10

Working quickly, pour the fudge into the prepared pan and smooth into an even layer. Scatter a few small dollops of marshmallow fluff over the top of the fudge, and swirl with a knife or spatula.

Step 11

Allow the fudge to cool for about 20 minutes at room temperature, and then place in the refrigerator to chill for about 4 hours, until solid.

Step 12

Remove from the refrigerator. If you've left the fudge in the refrigerator for up to 12 hours, it may be a bit too solid to slice without crumbling. Allow it to sit at room temperature until it can be sliced easily with a warm knife.

Step 13

Lift the fudge out of the pan by the overhung foil. Let a large, sharp knife run under warm water, and slice the fudge into 16 pieces. Moisten the knife under warm water after each cut.

Step 14

Serve immediately. Store leftover pieces in a sealed container in the refrigerator. This fudge will keep chilled for weeks.