In a bowl using a hand mixer, whisk egg whites until frothy on medium speed. Add cream of tartar and whisk until soft peaks. Gradually add sugar while whisking on high until stiff peaks.
Add vanilla and whisk until well incorporated.
Gently fold in the peanuts, making sure not to deflate the mixture. Place in a piping bag.
Line a tray with baking paper and pipe circles of the same size. Bake in an oven preheated to 100˚C until golden brown, about 20 minutes. Remove from pan and allow to cool thoroughly.
In a bowl, whisk egg yolks until pale and creamy.
Meanwhile, boil water and sugar together into a syrup.
Pour hot syrup in a thin stream over the egg yolks while whisking and continue until thick and cool to touch.
Add the slightly softened cubes of butter while whisking until all are incorporated.
Heat butter in a pot until melted.
Add the peanut butter and mix until incorporated. Next, add the sugar and salt. Remove from heat and let it cool.
Layer cooled meringue discs, buttercream, and peanut butter on top of each other.
Coat the entire cake with butter cream and sprinkle crushed peanuts.
Chill until firm.
