Melt the dark chocolate and butter in a heatproof bowl set over a pan of simmering water (double boiler). Let it cool to room temperature.
In a separate bowl, whisk the eggs and granulated sugar until light and fluffy. Gently fold in the melted chocolate mixture.
Sift together the flour, baking powder, and cocoa powder. Gradually add to the chocolate mixture, mixing until just combined.
Pour the batter into a greased and floured cake pan. Sprinkle half of the fresh blackberries over the batter. Bake in a preheated oven at 350°F (180°C) for 35-40 minutes, or until a wooden skewer inserted into the center comes out clean.
While the cake is cooling, whip the heavy cream with powdered sugar until stiff peaks form.
Once the cake has cooled completely, frost it with the whipped cream and decorate with the remaining blackberries. Dust with a little cocoa powder or grated chocolate, if desired.
Refrigerate the cake for at least 1-2 hours to allow the flavors to meld and the cream to set.
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