eight ounces (227 g) buttermilk (or regular milk with one Tbsp white vinegar added) (Make sure your buttermilk is fairly fresh, the older buttermilk is the higher the acidity level)
three ounces (eighty five g) vegetable oil three large eggs one tablespoon lime zest (About 1 lime)
one tablespoon lime juice eleven ounces (three hundred fifty g)
all-purpose flour eleven ounces (three hundred twelve g)
granulated sugar one teaspoon salt one teaspoon baking soda eight ounces (two hundred twenty seven g)
unsalted butter two tablespoons all-purpose flour (For dusting on blackberries)
four ounces (one hundred thirteen g) blackberries (You can use frozen, but don’t thaw them)
Blackberry Puree: sixteen ounces (four hundred fifty three g) fresh or frozen blackberries one tablespoon lime zest two tablespoon lime juice one tablespoon corn starch 1/8 teaspoon salt Blackberry Buttercream Frosting: twenty four ounces (six hundred sixty g) unsalted butter room temperature (You can use salted butter but it will affect the taste and you need to leave out additional salt) twenty four ounces (six hundred eighty g) powdered sugar sifted if not from a bag two teaspoons vanilla extract six ounces (one hundred seventy g) pasteurized egg whites room temperature two ounces (fifty seven g) blackberry puree (You can add more if you want to) 1 drop Americolor electric pink food coloring (Optional)
Blackberry Puree: Combine lime juice and cornstarch, to make a slurry, mix until no lumps. Mix with blackberries and salt in a saucepan. Use an immersion blender to make the filling smooth, turn on heat and being to cook down the berries, keep stirring to keep the puree from cooking too quickly on the bottom of the pan. Once puree has thickened up slightly, remove from heat stir in zest and transfer to a bowl to cool to room temp. You will use about six Tablespoons of the puree to add to your cake, the rest will be used to color and flavor your buttercream. You will end up with about 1/4-1/2 cup of puree for the buttercream. If you choose to strain the seeds out of all of the puree, add a couple extra ounces of berries to account for the loss in straining.
Once puree has thickened up slightly, remove from heat stir in zest and transfer to a bowl to cool to room temp. You will use about six Tablespoons of the puree to add to your cake, the rest will be used to color and flavor your buttercream. You will end up with about 1/4-1/2 cup of puree for the buttercream. If you choose to strain the seeds out of all of the puree, add a couple extra ounces of berries to account for the loss in straining. I chose to leave the seeds in for the cake, and strain the seeds out for the buttercream, seeds can be left in for the buttercream too, your choice. Store leftover puree in the fridge for up to a week or freeze for six months. Blackberry Cake: Heat oven to 335º F/168º C. Prepare two 8″ round cake pans with cake goop or another pan release that you prefer. Measure out buttermilk. Place four oz in a separate measuring cup. Add the oil to the 4oz of buttermilk and set aside.
To the remaining milk, add your eggs, lime zest, gently whisk to combine, keep the lime juice separate, you will add this at the end. Place the flour, sugar, baking soda, and salt into the bowl of your stand mixer with the paddle attachment. I’m using my Bosch universal plus mixer but you can use whatever mixer you have. Turn the mixer onto the slowest speed (setting 1 on Kitchen Aid mixers). Slowly add chunks of your softened butter until it is all added then let everything mix until it looks like coarse sand. Add your milk/oil mixture all at once to the dry ingredients and mix on medium (speed four on KitchenAid) for 1 full minute to develop the structure. Don’t worry, this will not over-mix the cake, because it is AP flour you do not need to mix as long as cake flour (more gluten, less time needed) Scrape the bowl. This is an important step. If you skip it, you will have hard lumps of flour and unmixed ingredients in your batter. If you do it later, they will not mix in fully.
Slowly add in the rest of your milk/eggs/zest ingredients in 3 parts, pour in the lime juice with the last addition of liquid, stopping to scrape the bowl one more time halfway through. Your batter should be thick and not too runny. Fill your pans half full with cake batter. Spoon about two tablespoons of room temp puree over the batter, use a small spatula or knife to gently fold the puree in to create a swirl effect into the batter. Add floured blackberries to the top of the batter, do not push them down in as their weight will cause them to sink anyway as the cake heats up. I always start by baking for thirty minutes for 6″ and 8″ cakes and thirty five minutes for 9″ and larger cakes and then checking for doneness. If the cakes are still really jiggly, I add another five minutes. I check every two minutes after that until I’m close and then it’s every one minute. Cakes are done when a toothpick inserted in the center comes out with a few crumbs. After cakes have cooled for ten minutes or the pans are cool enough to touch, flip the cakes over and remove from the pans onto the cooling racks to cool completely. Wrap in plastic wrap and chill in the freezer.
Once the cakes are chilled in the freezer and feel firm you can trim off the brown edges from the sides (optional) and fill with your blackberry buttercream and fresh berries. Crumb coat the cake with buttercream, chill then apply your final coat of buttercream. Top with fresh berries and lime zest.