Preheat and Prepare:

Preheat your oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper.

Make the Cake Batter:

In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.

In another bowl, whisk together the sugar, oil, eggs, milk, and vanilla extract until smooth.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Stir in the hot coffee or water (the batter will be thin — that’s normal).

Divide the batter evenly between the prepared pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Cool the cakes in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Blackberry Filling:

In a medium saucepan, combine the blackberries, sugar, and lemon juice over medium heat. Cook until the berries release their juices and soften (about 5 minutes).

Stir in the cornstarch mixture and cook until the filling thickens (2-3 minutes). Remove from heat and let it cool completely.

Make the Ganache Frosting:

Heat the heavy cream in a small saucepan until it just begins to simmer. Pour it over the chopped chocolate in a bowl. Let sit for 2 minutes, then stir until smooth and glossy. Add the butter, if using, and mix until fully incorporated.

Let the ganache cool until it thickens to a spreadable consistency.

Assemble the Cake:

Place one cake layer on a serving plate. Spread a layer of blackberry filling over the top, leaving a small border around the edges.

Add the second cake layer and spread the ganache frosting over the top and sides of the cake.