1. Make the Black Velvet Cake:

• Preheat Oven and Prepare Pans: Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans with nonstick cooking spray. Line the bottoms of the pans with circles of parchment paper. Grease the parchment paper and the sides of the pans.
• Combine Dry Ingredients: In a large bowl, sift together the all-purpose flour, black cocoa powder, granulated sugar, baking soda, baking powder, and salt. Whisk to combine thoroughly.
• Combine Wet Ingredients: In a separate medium bowl, whisk together the room-temperature buttermilk, canola oil, room-temperature eggs, and vanilla extract until well combined.
• Combine Wet and Dry Ingredients: Make a well in the center of the dry ingredients. Pour the wet ingredients into the well. Using a whisk or a rubber spatula, gently mix until just combined. Do not overmix.  
• Add Hot Coffee: Carefully pour the hot coffee into the batter and mix until just combined. The batter will be thin; this is normal.
• Divide Batter: Divide the batter evenly between the two prepared cake pans.
• Bake: Bake for 30 minutes, or until a wooden toothpick inserted into the center of each cake comes out clean (or with a few moist crumbs attached).
• Cool: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.

2. Make the Blackberry Compote:

• Combine Ingredients: In a medium saucepan, combine the fresh blackberries, granulated sugar, fresh lemon juice, lemon zest, and cinnamon stick.
• Simmer: Bring the mixture to a boil over medium heat, stirring occasionally.
• Reduce Heat: Once boiling, reduce the heat to low and simmer for 5-6 minutes, or until the blackberries have softened and released some of their juices.
• Make Cornstarch Slurry: In a small bowl, whisk together the water and cornstarch until smooth.
• Thicken Compote: Add the cornstarch slurry to the simmering blackberry mixture. Cook, stirring constantly, until the compote thickens to a jam-like consistency.
• Cool: Remove the cinnamon stick. Pour the compote into a small bowl, cover, and refrigerate for at least 1 hour, or until completely cool.

3. Make the Black Cocoa Frosting:

• Beat Cream Cheese and Butter: In a large bowl, using an electric mixer (hand mixer or stand mixer with the paddle attachment), beat the softened cream cheese and softened unsalted butter together until smooth and creamy.
• Sift Dry Ingredients: Sift together the powdered sugar, black cocoa powder, and salt into a separate bowl.
• Gradually Add Dry Ingredients: Gradually add the sifted dry ingredients to the cream cheese mixture, beating on low speed until just combined. Scrape down the sides of the bowl as needed.
• Add Vanilla: Add the vanilla extract and beat on medium speed for about 1 minute, until the frosting is light and fluffy.

4. Assemble the Cake:

• Level Cakes (Optional): If the cakes have domed tops, use a long serrated knife or a cake leveler to level them for easier stacking.
• First Layer: Place one cake layer on a cake plate or serving platter.
• Frosting Dam: Pipe or spread a ring of black cocoa frosting around the outer edge of the cake layer. This will create a “dam” to hold in the blackberry filling.
• Add Filling: Spoon the cooled blackberry compote inside the frosting ring, spreading it evenly.
• Second Layer: Carefully place the second cake layer on top of the filling, with the bottom of the cake facing up. This creates a flat surface for frosting.
• Crumb Coat: Apply a thin layer of the black cocoa frosting to the top and sides of the cake. This is called a “crumb coat” and it helps to seal in any loose crumbs.
• Chill: Refrigerate the crumb-coated cake for about 20 minutes to firm up the frosting.
• Final Frosting: Frost the chilled cake with the remaining black cocoa frosting, creating a smooth or textured finish, as desired.

5. Decorate (Optional):

• Decorate the cake with chocolate skulls, fresh blackberries, and dried rose petals (or other Halloween-themed decorations).