For the Cake:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.

2. Mix Dry Ingredients: In a medium bowl, sift together flour, black cocoa powder, cocoa powder, baking powder, baking soda, and salt.

3. Cream Butter and Sugar: In a large bowl, beat the softened butter and sugar together until light and fluffy, about 3-4 minutes.

4. Add Eggs and Flavoring: Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract and coconut oil (or olive oil).

5. Add Dry Ingredients and Buttermilk: Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined. Stir in the blackberry puree. If you want a deeper color, add a few drops of food coloring.

6. Bake: Divide the batter evenly between the prepared pans and smooth the tops. Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

For the Blackberry Filling:

1. Cook the Filling: In a medium saucepan, combine the blackberries, sugar, lemon juice, cornstarch, cinnamon stick, and water. Cook over medium heat, stirring occasionally, until the berries break down and the mixture thickens, about 8-10 minutes. Remove from heat and let cool. Discard the cinnamon stick and strain the filling to remove any remaining seeds.

Assemble the Cake:

1. Layer the Cake: Once the cakes have cooled completely, place one cake layer on a serving plate. Spread a generous layer of blackberry filling on top.

2. Add the Second Layer: Place the second cake layer on top and press down gently.

3. Frost (optional): You can frost the cake with whipped cream, buttercream, or cream cheese frosting to complement the blackberry filling. Optionally, garnish with fresh blackberries or a dusting of powdered sugar.