Step 1: Prepare the Cake Layers

1. Preheat the oven to 350°F (175°C). Grease and line two 8-inch cake pans with parchment paper.
2. In a large bowl, sift together the flour, black cocoa powder, sugar, baking powder, baking soda, and salt.
3. In another bowl, whisk the buttermilk, oil, eggs, and vanilla extract until well combined.
4. Create a well in the dry ingredients and pour in the wet mixture. Stir until just combined, then mix in the hot coffee until the batter is smooth.
5. Divide the batter evenly between the prepared pans and bake for 30-33 minutes, or until a toothpick inserted into the center comes out clean.
6. Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Step 2: Make the Blackberry Compote

1. In a medium saucepan, combine the blackberries, sugar, lemon juice, lemon zest, and cinnamon stick. Bring to a boil, stirring occasionally, and cook for 5-6 minutes.
2. Stir the cornstarch mixture into the blackberries and cook until the compote thickens. Remove the cinnamon stick and transfer the compote to a bowl to cool in the refrigerator for 1 hour.

Step 3: Prepare the Buttercream

1. In a large bowl, beat the softened cream cheese and butter together with a hand mixer until smooth.
2. Gradually sift in the powdered sugar, black cocoa powder, and salt. Beat until creamy and fully combined. Add the vanilla extract and beat for another 1-2 minutes.

Step 4: Assemble the Cake

1. Once the cake layers are completely cool, place one layer on a serving plate or cake stand.
2. Spread a layer of blackberry compote over the cake, leaving a small border around the edges.
3. Place the second cake layer on top and press gently.
4. Frost the entire cake with the black cocoa buttercream, smoothing it out with a spatula.

Step 5: Decorate

1. Top the cake with fresh blackberries and chocolate skulls or other spooky decorations for a Halloween-themed touch.