Preheat your oven to 350°F (175°C), and prepare two 8-inch or 9-inch cake pans by greasing and lining them with parchment paper for easy release.

Whisk together the dry ingredients in a large bowl: all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until evenly combined.

Combine in another bowl the wet ingredients: eggs, vegetable oil, buttermilk, and vanilla extract. Mix until well combined, then gently blend this mixture with the dry ingredients. Finally, stir in the hot water to create a smooth batter.

Divide the batter equally between the prepared pans, and bake for 30-35 minutes. The cakes are ready when a toothpick inserted into the center comes out clean. Cool the cakes in their pans for 10 minutes before transferring to a wire rack to cool completely.

Prepare the filling by cooking the blackberries with granulated sugar in a saucepan over medium heat until they are juicy. Then, add the cornstarch slurry and lemon juice. Stir while heating until the mixture thickens, then remove from heat and let cool.

Whip the heavy cream with powdered sugar and vanilla extract in a mixing bowl until stiff peaks form, creating a light and fluffy topping.

Assemble the cake by placing a layer on a serving plate, spreading the blackberry filling on top, and then adding the second layer. Finally, top off with the whipped cream and garnish, if desired, with extra blackberries, chocolate shavings, or edible flowers.

Optional: Dust with cocoa powder for an elegant finish.