Preheat the oven to 350°F (180°C) and line a 9-inch (22 cm) cake pan with parchment paper.
Melt the dark chocolate over a double boiler, then let it cool slightly.
In a bowl, whisk together the flour, baking powder, and cocoa powder.
In another bowl, cream together the butter and sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition.
Slowly stir in the melted chocolate, then gradually add the dry ingredients until just combined.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean. Let it cool completely in the pan.
Make the mousse: whip the heavy cream to stiff peaks. Gently fold in the melted chocolate and powdered sugar.
Slice the cooled cake horizontally into two layers and spread the chocolate mousse between the layers.
Frost the cake with the remaining mousse and garnish with cocoa powder or chocolate shavings.
Refrigerate the cake for at least 2 hours before serving to allow the mousse to set.
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