In a medium heavy bottom saucepan over medium-high heat, combine water, sugar, and corn syrup and stir to combine.
Insert candy thermometer, and set for 240°F (if yours has a beeper). Do not stir while heating.
While sugar mixture is heating, place room temperature egg whites and cream of tartar into a large mixing bowl. Beat until soft peaks form, about 3-4 minutes.
When the sugar syrup mixture reaches 240°F, remove from heat immediately.
With your mixer running, carefully pour syrup in a thin steady stream into the whipped egg whites, beating the whole time. The whites will deflate at first, but as the sugar syrup becomes incorporated, it will thicken, turn white, and fluff up. Whip for about 7-8 minutes until fluffy.
Add vanilla and whip until incorporated.
Use immediately or place marshmallow creme in a sealed container and store in the refrigerator,
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Notes
This recipe makes about 6 cups. 1 serving in the nutrition facts is about ¼ cup.
Make sure the bowl you use to whip the egg whites is very clean and grease-free or you egg whites may not whip up properly.
If you're worried about the egg whites, you can substitute 3 tablespoons meringue powder and 5 tablespoons of water.
Instead of corn syrup, you can use honey, glucose syrup, or rice malt syrup. These may change the color of of your fluff.
Don't stir the sugar and water mixture while it's heating because it causes the sugar to form crystals. This will make your fluff a bit gritty.
Storage: Store in an air tight container in the refrigerator for up to 2 weeks. You may need to whip again before use to achieve the lighter consistency.
Freezer Directions: Freeze in a freezer bag or container for up to 3 months. Thaw overnight in the refrigerator. Place in a large mixing bowl and beat before use to re-fluff.