• Read the recipe notes and the post for helpful information regarding this recipe.

Make the Filling

• Put the filling ingredients in a medium-sized saucepan on medium heat and cover. Allow the berries to defrost and cook for about 10 minutes. Then use an immersion blender to carefully blend everything until there are no whole berries remain. If you don’t own an immersion blender, use a potato masher to mash everything up instead.
  
• Once all your berries have been blended or mashed, strain through a fine mesh strainer to remove all the seeds. Return your liquid to the pot and cook on medium, stirring frequently for around 25-30 minutes until your filling has halved in quantity and reached a loose, jammy consistency. It will continue to set as it cools. You want your filling to be thick, that way when you add it to the frosting, it doesn’t water it down.
  
• Remove from the heat andallow it to cool completely as you make your cake and frosting.

Make the Cake

• Preheat oven to 350 degrees F.
• Grease three 8-inch round cake pans with nonstick spray or softened butter and coat the bottom and sides of the pans with gluten free flour. Then line the bottoms with parchment paper that has been cut to size.
  
• Sift together your dry ingredients into a medium bowl and set aside.
  
• Then, add the milk and lemon juice to a small bowl and set aside.
  
• In a large bowl, add the granulated sugar and lemon zest. Use your fingers to combine and massage the zest into the sugar to release the lemon oils from the zest and give the cake a more lemon flavor.
  
• Then whisk in the oil, melted butter, eggs, and vanilla extract until everything is combined.
  
• Pour in half of the dry ingredients and mix well.
  
• Then add in the milk mixture and stir to combine.
  
• Add in the last of the dry ingredients. Stir until everything is fully incorporated and there are no lumps.
  
• Divide the batter evenly between the three prepared cake pans and bake for 18-22 minutes or until a toothpick comes out clean.
  
• When the cake layers are done, allow them to cool for 10 minutes in the pans then invert them onto wire cooling racks. Let the cakes rest and cool completely before assembling.
  

Make the Frosting

• Add the butter, vanilla extract, and ⅓ cup of the cooled blackberry filling to the bowl of a stand mixer. Use the paddle attachment to beat, on medium-low speed, until the butter and blackberry are thoroughly combined.
• Sift in half of the powdered sugar and mix on low speed. Then scrape down the bowl and sift in the remaining sugar. Mix until combined.
• Once all the sugar is incorporated with the butter, increase the speed and whip until the buttercream is smooth and light. If the buttercream is too stiff, mix in 1 tablespoon of the blackberry filling at a time. If it’s too loose, add a small amount of powdered sugar until it reaches your desired consistency.

Assemble

• If the cake layers aren’t flat, use a serrated knife to level them off. Then, add a small dollop of icing in the middle of your cake plate or stand to secure the first cake layer.
• Use an offset spatula to spread a thin layer of icing over the top of the cake. This will help protect the cake from getting soggy from the filling.
• Pipe or spoon a border of icing around the rim of your cake layer to keep any filling from seeping out. Dollop on about ⅓ of a cup of the blackberry filling and spread it evenly to fill the cake. Place the next cake layer on top, being careful to center it, and repeat the same process for the second layer.
  
• Add the final cake layer and frost the cake with a crumb coat to seal in the filling and any cake crumbs. Refrigerate for 15-30 minutes before frosting. Once the crumb coat is set, frost your cake with the remaining frosting.
  
• Garnish with fresh blackberries, lemon zest, or lemon wheels, and enjoy!