Place water and sugar in a small saucepan over medium heat. Have a thermometer ready.
Put the egg and egg yolks in the bowl of a stand mixer fitted with the whip attachment. Turn the mixture to high speed and let whip until the eggs become thick and hold their shape.
When the sugar syrup reaches 113°C (235°F) turn the mixer to medium speed, then take the saucepan off the heat and gradually pour the syrup into the mixing bowl.
Turn the mixer back up to high speed and whip until the side of the bowl feels cool to the touch. The meringue may not be stiff like a traditional meringue at this point - don't worry. As long as it's cool, it'll come together.
Once the mixture is cool, remove the whip and replace with the paddle attachment. Turn the mixer back on to medium, then add the small cubes of butter, one at a time. Continue beating and adding the butter until it’s all added and smooth.
When done, switch the speed to low and add the peanut butter, salt, and vanilla. Taste, and add more peanut butter a tablespoon at a time, if desired, until you find your perfect salty-sweet balance. Feel free to add more salt to taste as well.
Use peanut butter that's darker in colour - the peanuts are usually roasted for longer before grinding, yielding better flavour.