Heat oven to 350 degree F.
Grease and flour 4 x 7 inch cake pans, line bottom with parchment. Set aside.
Combine sugar, flour, cocoa, baking powder, baking soda and salt in bowl of mixer.
Add in the eggs, milk, oil and vanilla, beat medium speed for 2 minutes. Scrape sides of bowl as needed.
With mixer on medium speed (reduce to low speed if too much splashing occurs) slowly stream in the boiling water. Continue to mix until completely combined. Scrape sides and bottom of bowl as needed. Batter will be watery.
Pour into prepared cake pans, only fill pans 1/2 way, and bake 30-35 minutes or until toothpick inserted comes out clean.
Let cool completely.
BUTTERCREAM:In large sauce pan add the egg whites and sugar.On medium heat you are going to whisk egg mixture until sugar is completely dissolved (you can test it by rubbing a small amount between your fingers to test it, it should be completely smooth).Transfer to mixing bowl and with whisk attachment whisk on high until stiff peaks form and mixture is completely cooled (if not completely cooled your butter will melt). You may have to let mixture sit for 20 minutes until completely cooled. On low speed add the butter in small chunks one piece at a time.Bring mixture up to high speed and continue to mix until well combined, scraping down sides of bowl as needed. Add in the vanilla extract and salt, mix to combine.For the raspberry buttercream: Add in the raspberrie puree now, mix well to combine, scraping down sides of bowl as needed. For the chocolate buttercream: Add in the chocolate now, mix well to combine, scraping down sides of the bowl as needed.
Cut any humps off each cake layer that may have formed in baking.
Place first cake layer on cake board, secure it in place with a dab of frosting underneath.
Spread even layer of butter cream on top of the cake layer smooth with offset spatula.
Repeat with the next 2 cake layers. From the pictures you will see I used 2 layers of raspberry buttercream filling and 1 layer of chocolate filling.
Add the top cake layer and crumb coat the cake with buttercream.
Refrigerate at least 60 minutes.
Add final coat of raspberry buttercream. If adding the chocolate buttercream bottom refrigerate cake 60 minutes, then proceed with spreading on the chocolate buttercream as high up as you would like to go and then using an offset spatula or scraper smooth it out.
Keep in cool place and bring to room temp before serving.