Place the egg whites and sugar in a large metal mixing bowl and whisk together.
Set the bowl over a small pot of simmering water, and heat, whisking occasionally, until the mixture becomes translucent and no longer feels gritty when rubbed between the thumb and forefinger.
Remove from heat and whip on high speed until the meringue becomes glossy and thick.
Cool completely (there should be no hint of warmth when the hand is place on the side of the bowl. Place in the refrigerator for 10-20 minutes, if necessary.)
Add the butter, a tablespoon at a time, while whipping on medium-high.
When all the butter has been added, fold in the peanut butter, vanilla, and salt.
Transfer the buttercream to a pastry bag fitted with a 4B tip.
Pipe swirls of frosting onto the cupcakes, top with a drizzle of salted caramel sauce, and sprinkle with peanuts.