Step 1: Preheat your oven to 180°C (350°F). Grease and flour two 8-inch round cake tins.
Step 2: For the sponge, mix all dry ingredients (flour, cocoa powder, baking powder, baking soda) in a large bowl. Set aside.
Step 3: In a separate bowl, whisk together the eggs, sugar, oil, melted dark chocolate, buttermilk, and vanilla extract until well combined and smooth.
Step 4: Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Be careful not to overmix.
Step 5: Divide the batter evenly between the two prepared tins. Bake for 25-30 minutes, or until a skewer inserted into the center comes out clean. Let the sponges cool in the tins for 10 minutes before inverting them onto a wire rack to cool completely.
Step 6: While the sponges cool, make the blackberry syrup. In a small saucepan, combine 200 g blackberries, 70 g sugar, and 1 tbsp lemon juice. Simmer gently for 10 minutes, stirring occasionally, until the berries are soft and the syrup has thickened slightly.
Step 7: Strain the blackberry syrup through a fine-mesh sieve, pressing on the solids to extract all the liquid. Discard the solids. Once the sponge layers are completely cool, brush generously with the blackberry syrup.
Step 8: Prepare the blackberry cream. In a saucepan, cook 150 g blackberries with 2 tbsp sugar over medium heat until the berries are soft and bursting. Blend the cooked blackberries until smooth, then strain through a fine-mesh sieve to remove seeds. Set aside.
Step 9: Melt 150 g dark chocolate. In a separate bowl, warm 100 ml of the double cream (do not boil). Gradually mix the warm cream into the melted chocolate until smooth and glossy. Stir in the strained blackberry puree.
Step 10: In a separate cold bowl, whip the remaining 150 ml double cream until soft peaks form. Gently fold the whipped cream into the chocolate blackberry mixture until fully incorporated. Chill the cream in the refrigerator for about 30 minutes to set slightly and become spreadable.
Step 11: For the topping, combine 250 g blackberries, 80 g sugar, and 1 tsp lemon juice in a saucepan. Simmer over medium heat until the berries break down and the mixture thickens. If desired, mix 1 tsp cornflour with a tablespoon of cold water and stir into the simmering berries to achieve a thicker, glossier consistency. Cook for another minute, then remove from heat and let cool.
Step 12: Assemble the torte. Place one sponge layer on your serving plate. Spread half of the blackberry cream evenly over the sponge. Carefully place the second sponge layer on top. Spread the remaining blackberry cream over the second layer.
Step 13: Pour the cooled blackberry topping over the center of the torte, allowing it to drip naturally down the sides for a rustic look. Arrange fresh blackberries and mint leaves artfully on top for garnish.
Step 14: Refrigerate the torte for at least 2-3 hours (or overnight) to allow it to set completely and for the flavors to fully develop. Slice and serve chilled.