Preheat the oven to 325 degrees F.  Generously mist three 8-inch diameter cake pans with non-stick spray, and line with circles cut from parchment paper.

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

Cut the butter into smaller pieces and add it to the dry ingredients.   Mix on low speed until the mixture resembles damp sand (30 seconds to a minute).  

Add the eggs, one at a time, scraping the bottom and sides of the bowl with a silicone spatula after each addition.  

When all of the eggs are fully incorporated, add the Greek yogurt and vanilla, and beat on medium speed for about 60 to 90 seconds, to aerate the batter and strengthen the cake's structure.

Divide the batter evenly between the three prepared pans, and bake for 30-40 minutes, or until a toothpick inserted in the thickest part of the cake comes out clean or with one or two moist crumbs.  Cool completely, then fill and frost with raspberry buttercream.

Puree the thawed berries in a food processor, then strain the mixture through a fine mesh sieve, discarding the seeds.  

Place the puree in a small pot and cook on medium-low heat until thickened and reduced by about half.  You should have about 1/4 cup of cooked puree.  Set aside to cool.

Place the egg whites and sugar in a large mixing bowl set over a pot of simmering water.  

Cook, whisking occasionally, until the mixture is smooth, thin, and translucent, and it no longer feels gritty when rubbed between the thumb and forefinger.  

Remove the bowl from the heat and whisk on medium-high speed until fluffy and stiff, and no hint of warmth remains when you place your hand on the side of the bowl.  

Add the butter, a tablespoon at a time, whipping after each addition.  If the mixture seems runny, place it in the refrigerator for 10 to 20 minutes, then re-whip.  When all the butter has been added and the mixture is fluffy and spreadable, stir in the cooled raspberry puree until fully incorporated.