Ingredients:
Blackberry Cake:
8 ounces (227 g) buttermilk (or regular milk with 1 Tbsp white vinegar added) (Make sure your buttermilk is fairly fresh, the older buttermilk is the higher the acidity level)
3 ounces (85 g) vegetable oil
3 large eggs
1 tablespoon lime zest (About 1 lime)
1 tablespoon lime juice
12 ounces (340 g) all-purpose flour
11 ounces (312 g) granulated sugar
1 teaspoon salt
1 teaspoon baking soda
8 ounces (227 g) unsalted butter
2 tablespoons all-purpose flour (For dusting on blackberries)
4 ounces (113 g) blackberries (You can use frozen, but don't thaw them)
Blackberry Puree:
16 ounces (453 g) fresh or frozen blackberries
1 tablespoon lime zest
2 tablespoon lime juice
1 tablespoon corn starch
1/8 teaspoon salt
Blackberry Buttercream Frosting:
24 ounces (680 g) unsalted butter room temperature (You can use salted butter but it will affect the taste and you need to leave out additional salt)
24 ounces (680 g) powdered sugar sifted if not from a bag
2 teaspoons vanilla extract
6 ounces (170 g) pasteurized egg whites room temperature
2 ounces (57 g) blackberry puree (You can add more if you want to)
1 drop Americolor electric pink food coloring (Optional)
Instructions:
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 6 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 6 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 4 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 4 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 1/2 full with cake batter.