Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.
Break the dark chocolate into pieces and melt it over a double boiler or in the microwave.
Cream together the butter and sugar until light and fluffy, then add the egg yolks one at a time.
Stir in the melted chocolate, then add the flour, baking powder, and cocoa powder. Mix until just combined.
In a separate bowl, whisk the egg whites until stiff peaks form. Gently fold the egg whites into the chocolate mixture.
Pour the batter into the prepared baking sheet and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let it cool completely.
Meanwhile, whip the heavy cream until stiff peaks form. Spread the whipped cream evenly over the cooled cake.
Dust the top with powdered sugar and cocoa powder. Slice and serve.
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