For the Black Velvet Cake:

1. Preheat the oven: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, or line them with parchment paper for easy removal.
2. Make the cake batter: In a large bowl, sift together the flour, sugar, black cocoa powder, baking powder, baking soda, and salt. In a separate bowl, whisk together the eggs, milk, oil, and vanilla extract. Add the wet ingredients to the dry ingredients and stir until just combined.
3. Add the boiling water: Slowly pour the boiling water into the cake batter while mixing on low speed. The batter will be thin—this is normal! Mix until fully combined and smooth.
4. Bake: Divide the batter evenly between the two prepared cake pans. Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

For the Blackberry Compote:

1. Cook the blackberries: In a small saucepan over medium heat, combine the blackberries, sugar, and lemon juice. Cook until the berries start to break down, about 5–7 minutes.
2. Thicken the compote: Stir in the cornstarch mixture and cook for another 2–3 minutes, until the compote thickens. Remove from heat and let cool completely before using.

For the Black Cocoa Buttercream Frosting:

1. Beat the butter: In a large bowl, beat the softened butter on medium speed until creamy, about 2 minutes.
2. Add powdered sugar and cocoa: Gradually add the powdered sugar and black cocoa powder, 1 cup at a time, mixing on low speed. Once combined, increase the speed and beat until light and fluffy.
3. Add cream and vanilla: Pour in the heavy cream and vanilla extract, then beat for another 2–3 minutes until the frosting is smooth. Add a pinch of salt to balance the sweetness.

Assemble the Cake:

1. Level the cake layers: If necessary, use a serrated knife to level the tops of the cake layers so they are flat and even.
2. Fill with blackberry compote: Place one cake layer on a serving plate or cake stand. Spread a generous layer of blackberry compote over the top, leaving about 1/2 inch around the edges to prevent it from spilling out.
3. Frost the cake: Place the second cake layer on top of the compote. Frost the entire cake with the black cocoa buttercream, starting with a thin crumb coat and then adding a thicker layer of frosting.
4. Decorate: Top the cake with fresh blackberries and chocolate skulls (or any other spooky Halloween decorations you like).
5. Serve: Slice and enjoy this rich, decadent black velvet cake!