Preheat your oven to 350°F (180°C). Line a 9-inch (22 cm) cake pan with parchment paper.
Melt the dark chocolate and butter together in a heatproof bowl set over a pan of simmering water (double boiler). Let it cool slightly.
In a separate bowl, whisk the eggs and sugar until light and fluffy. Gradually add the melted chocolate and butter mixture, whisking until combined.
In another bowl, combine the ground walnuts, flour, and baking powder. Gently fold the dry ingredients into the egg mixture until just combined. Be careful not to overmix.
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan before removing it.
In a chilled bowl, whip the heavy cream with the powdered sugar until stiff peaks form.
Using a long serrated knife, carefully slice the cake horizontally into two layers. Spread the whipped cream evenly over the bottom layer, then gently place the top layer over the cream.
Drizzle with melted chocolate and sprinkle with toasted walnuts for decoration. Refrigerate for at least 1 hour before serving.
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