Combine egg whites and granulated sugar in the heatproof bowl of a standing mixer.
Set the mixer bowl over a small pan of simmering water and whisk egg mixture until warm to the touch and completely smooth (check with your fingers to be sure all the sugar has dissolved).
Remove mixer bowl from pan and attach to your standing mixer.
Turn your mixer on low and gradually up to high speed with the whisk attachment.
Continue mixing for about 8-10 minutes, or until the bottom of the mixing bowl is cool to the touch.
The egg mixture should be glossy white and fluffy and COOL!
Add in butter, 1-2 tablespoons at a time, until completely mixed in.
Whisk in vanilla extract.
In a double boiler (or heatproof bowl over simmering pan of water), heat 1 cup of peanut butter chips until soft and you can stir them into a paste.
Remove from heat and let cool slightly.
Whisk peanut butter paste into buttercream until completely mixed.
Switch to the paddle attachment and continue mixing buttercream for 2-3 minutes more.
If frosting is too soft to pipe, place in refrigerator for a little while to help firm it up.