Cake

1. Preheat the oven to 350F/180C and heavily grease two 9-inch cake pans, and set them aside. Your oven must be properly preheated for your cake to bake properly.
2. In a bowl of an electric stand mixer fitted with a paddle attachment or a large bowl with an electric handheld mixer combine the cake flour, baking powder, baking soda, salt, lemon zest, and granulated sugar. Once fully combined for 30-60 seconds add the softened butter in chunks and mix until incorporated and the mixture becomes like wet sand.
3. In a bowl combine the buttermilk, sour cream, oil, and vanilla extract then add to dry ingredients and mix for 60 seconds, you want to really incorporate everything to make sure the batter builds structure. Add in the eggs and mix for an additional 45-60 seconds, there should be no trace of egg left. Use a rubber spatula to scrape the bottom of the bowl to make sure everything is evenly combined. In a small bowl combine the blackberries and flour. Gently mix in 3/4 of the berries into the cake batter. Pour cake batter into prepared pans and top with remaining blackberries scattered on top, gently press into cake batter ever so slightly.
4. Quickly and gently place the pans in the oven and do not open the oven door until at least the 20-minute mark, if you have an oven light use it! Do this very quickly and don't close the door too abruptly. Your cakes should be done baking in 25-30 minutes. If you press a toothpick into the center it should come out with no wet batter, or you can gently press the top center of the cake and it should bounce back immediately, the sides of the cake will be coming away from the edges of the pan. Set on a wire rack to cool for 30 minutes before inverting onto the wire rack to cool completely before decorating.

Blackbery Filling

1. While the cake is baking. In a small bowl whisk together the cornstarch and lemon juice to create a slurry.
2. In a large pot add all ingredients and cook over low-medium heat for 4-5 minutes, stirring frequently. You want to bring the mixture to a slight boil then, reduce the heat and continue to stir. The berries will naturally sort of break down. The longer you cook it the more reduced it will become. Pour into a heat proof container.
3. Let cool for 30 minutes then store in the fridge to chill.

Frosting

1. Once the cakes have cooled and the blackberry filling has chilled. In the bowl of an electric mixer fitted with a paddle attachment, or a large bowl with a handheld mixer beat the butter for 5 full minutes, until light and fluffy.
2. Sieve the powdered sugar to prevent any lumps and add half to butter. Beat for another 5 minutes then add the remaining powdered sugar and mix for 3 more minutes. Make sure to scrape down the sides of the bowl multiple times to ensure everything is evenly combined.
3. Add the vanilla extract and heavy cream and beat for another 2 minutes. Mix in the 4 tablespoons of blackberry filling and scrape the bowl to ensure no white spots remain.
4. Using a serrated knife saw off the dome of the cake to create a flatter, even surface. Place the bottom layer on the serving plate and place a large dollop of frosting on the cake then use a small angled spatula to spread an even layer of frosting onto the first cake layer. Then pipe a border of frosting to create a dam for the filling. Spread the 3/4 of the blackberry filling into the cake center and spread into an even layer, making sure not to go over the frosting dam. Make sure to get down and look at the cake at eye level to see if it is level or not, and adjust as necessary. Place the second cake layer on top, add a thin layer of frosting around the whole cake to create a crumb coat, then place the cake in the fridge for 30 minutes or freezer for 10 minutes to set it. Frost with remaining frosting and add blackberries to the cake.
5. Store the cake in an air-tight container in the refrigerator for up to 5 days, although best tasting the first two days. For best results, let the cake come to room temperature for 10-15 minutes before enjoying a slice.