Preheat the oven to 350 degrees F, and line a cupcake pan with papers.

Place the flours, sugar, baking powder, baking soda, and salt in a large mixing bowl and whisk to combine.  

Add the butter, and mix on medium-low speed, until the mixture resembles damp sand.  

Stir in the peanut butter, then add the eggs one at a time, mixing until fully incorporated.  

Scrape the bottom and sides of the bowl with a silicone spatula, then add the Greek yogurt, milk, and vanilla.  

Mix on medium-high speed for approximately 90 seconds, to aerate the batter and build the cake's structure.

Place about 1/4 cup of batter in each well of the cupcake pan.  

Bake for 14-16 minutes, or until a toothpick inserted in the thickest part of the cake comes out clean or with a few moist crumbs.  

Cool completely, then cut a small well in each and fill with grape jam.  

Top with buttercream and drizzle with more grape jam.

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.