Combine the flour, sugar, cocoa, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with a paddle attachment and mix on low speed until combined.

In another bowl, combine the buttermilk, oil, eggs, water and vanilla.

With the mixer on low speed, slowly add the wet ingredients to the dry, scraping the bottom of the bowl with a rubber spatula.

Cool in the pans for 10 minutes, then turn them out onto a cooling rack and cool completely.

Wrap in plastic wrap and chill until ready to use. You're still going to want to level your cake layers to ensure a completely even surface for frosting and stacking.

Combine the blackberries and sugar in a medium saucepan over medium heat. Stir the mixture while bringing it to a boil and occasionally breaking up the berries to release their juices.

Add the cornstarch.

Let simmer for about 10 minutes until the mixture is reduced by half. (We're trying to reduce the amount of water and thicken the mixture.)

Strain 1/2 cup of the mixture and reserve for frosting. Refrigerate the strained blackberry mixture and the remaining compote for at least 4 hours before using. 

In the bowl of a stand mixer fitted with the paddle attachment cream the butter on medium speed for about 3 minutes, until the mixture is light and fluffy.

With the mixer on low speed, slowly add the powdered sugar, followed by the blackberry juice, heavy cream, vanilla extract and dash of salt.

Continue to beat the frosting on medium-high speed for an additional 5 minutes.    

Before frosting your cake, mix the frosting by hand with a wooden spoon to push out the air pockets. This will also help the frosting pull back together if it looks separated from adding the blackberry juices.

Spread a dollop of frosting on a cake board or cake plate to hold the first cake layer in place.

Place the first cake layer, top side up, on the cake board and pipe a rim of blackberry frosting around the edges to act as a barrier to the compote. For the 6-inch round cake, spread about 1/4 cup blackberry compote on the cake layer. For an 8-inch cake, spread 1/2 cup blackberry compote on the cake layer.

Top with the next cake layer and repeat step 2.

Place the final cake layer top side down. Apply a thin layer of frosting around the entire cake to lock in the crumbs. Freeze the cake for about 10 to 15 minutes to set the crumb coat.

If your frosting feels soft at this point, refrigerate it while the crumb coat sets.

Continue frosting your cake, making sure to stir the frosting with a wooden spoon to push out any air pockets.

If you're not serving within the day, you can freeze the cake in a cake box or container for up to a week - just make sure to remove the cake from the freezer about 6 hours before you plan to serve it.