Preheat your oven to 350°F (180°C). Line a 9-inch (22cm) cake pan with parchment paper.
Melt the dark chocolate and butter in a heatproof bowl set over a pan of simmering water (or in the microwave). Let it cool slightly.
In a large bowl, whisk the eggs and sugar until light and fluffy. Stir in the vanilla extract and the melted chocolate mixture.
In a separate bowl, whisk together the flour and cocoa powder. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let the cake cool completely in the pan.
In a chilled bowl, whip the heavy cream and powdered sugar until stiff peaks form. Gently fold in the blackberry jam.
Using a long serrated knife, slice the cake horizontally into two layers. Spread the blackberry cream over the bottom layer, then top with the second cake layer.
Frost the top of the cake with the remaining cream and decorate with fresh blackberries. Refrigerate for at least 1 hour before serving to allow the flavors to meld.
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