To prepare the cake, preheat the oven to 350 and prepare three 6” round cake pan with cooking spray and baking parchment.
In a large bowl, whisk together the flour, sugar, cocoa, brown sugar, baking powder, and baking soda, and set aside.
In another bowl, or a large jug whisk together the buttermilk, oil, vanilla and eggs.
Whisk the wet ingredients into the dry, then whisk in the boiling water to lighten the batter.
Divide the batter between the prepared cake pans, and smooth the tops, dropping them down on the countertop a few times to knock out any large air bubbles. Bake for 30 minutes, or until a toothpick inserted into the middle of each cake layer comes out clean.
Allow the cakes to cool in the pan until it is cool enough to touch, then turn them out onto a wire rack to cool completely.
While the cake is cooling, make the frosting. In a stand mixer fitted with the whisk attachment, or using a hand held electric whisk, beat together the softened cream cheese and butter until they are combined, light and smooth. Then beat in the vanilla.
Sift in the cocoa, mix with a spatula so they’re just combined (this will stop the cocoa flying up at you when you whisk!) before beating into the butter mixture using the stand mixer or whisk until smooth. Repeat, scraping down the sides each time with the powdered sugar, adding it roughly 1/2 a cup at a time. You should be left with a light, smooth and tangy frosting.
Before you assemble the cakes, level them off with a serrated knife or cake leveller until they’re all of an even thickness.
Load 1/4 of the frosting into a piping bag, and pipe a thick, almost 1 inch border of frosting around the edge of the bottom layer of cake. Fill the middle with some of the raspberry cake filling, place the next cake layer on top and repeat. Don’t worry if you don’t use all the cake filling. Place the final cake layer on top, cut size down to make it easier to frost.
Using a palette knife, coat the cake in a ‘crumb coat’ using the remaining frosting from the piping bag, plus just over half of what is leftover. The idea is to seal the cake and provide a smooth layer of frosting for the finishing coat. Chill for 30 minutes so the frosting can firm up.
Smooth the rest of the frosting over the chilled cake, and chill for a further 15 minutes before slicing and decorating with additional raspberries, if you have any fresh ones leftover from making the cake filling.