Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.
Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.
In a medium bowl whisk all wet ingredients (pour water in slowly as not to cook the eggs if very hot).
Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.
Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed.
Bake until a cake tester comes out mostly clean. A total of 35-40mins.
Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.
Peanut Butter Swiss Meringue Buttercream:
Place egg whites and dark brown sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**
Add powdered peanut butter 1 Tbsp at a time. Add vanilla and whip until smooth.
Place one layer of cake on a cake stand or serving dish. Top with about 2/3 cup buttercream. Repeat with remaining layers. Frost and smooth the outside with a thin crumb coat (thicker on the very top). Chill for 20mins.
Do a decorative rope border on top using a 1M tip if desired.***
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. *** This recipe makes enough frosting to fully frost the cake if desired.