In a medium saucepan, add in the sugar, water, and kosher salt and cook over medium heat; stirring occasionally (use a metal spoon) until the syrup is translucent and no visible granules of sugar can be seen when lifting the spoon out of the syrup. This takes about 4-5 minutes.
Remove from heat, stir in the vanilla, and set aside to cool slightly.
Crack those eggs and separate the egg whites into a small bowl (save those yolks for something else).
Check for any shells in the whites, then transfer them to your stand mixer.
Using the whipping attachment, beat the eggs on low speed until the eggs look foamy, about two minutes.
Increase the speed to medium and continue to beat until soft peaks form (see image earlier in the post for an example. If you take the bowl, tip it to the side, and the whites stay put, you succeeded).
Slowly add in the warm syrup starting with 1-2 tablespoons at a time. After you're half done, slowly pour the remaining syrup into the mixture.
Increase the speed to medium-high and let it go for five minutes. Set a timer and grab yourself a drink! The marshmallow will turn into this fluffy deliciousness doubled in size.
Store in an airtight container for up to two weeks.*