Mix the egg whites and sugar in a mixing bowl. Set the bowl over a pan of simmering water and stir constantly until the sugar is dissolved.
Whisk the warm mixture until stiff then 6 more minutes on high.
Switch to the paddle attachment. Add the butter in batches while beating on medium-low making sure each batch is well blended.
Add the vanilla extract and beat on medium-high for 3-5 minutes until smooth. Beat on low for another 2 minutes to remove any air bubbles
To use as a filling you can stir in flavorings at the end with a rubber spatula. Some options are:
Lemon: add 3/4 cup of lemon curd
Strawberry or other red berries: add 1 1/4 cup of pureed jam