4 large egg whites
1 tsp cream of tartar
1/8 tsp salt
2/3 c + 3 Tbsp sugar, divided
3/4 c light corn syrup
1 tsp vanilla extract
In the bowl of a standing mixer fitted with whisk attachment, whisk egg whites vigorously for 1 minute. Sprinkle with cream of tartar and salt; continue beating until foamy. Continue beating while sprinkling in 3 Tbsp sugar. Beat until soft peaks form.
In a medium saucepan, gently stir together remaining 3/4 c sugar, 1/4 c water and corn syrup. Set saucepan over low heat; stir until sugar and syrup dissolve. Turn heat up to medium; bring to a boil. Clip a candy thermometer to the side, turn heat up to medium-high; and wait for mixture to reach firm ball stage (246-248 degrees), about 10 minutes.
Turn mixer to low. Slowly stream hot sugar into egg whites. Once all sugar is added, increase mixer speed to high. Beat until marshmallow cream is near room temperature and fluffy. Add vanilla; beat 5 more seconds or until incorporated. Use immediately or refrigerate tightly covered for up to 3 days.