Preheat your oven to 350°F (180°C). Prepare a cake pan by lining the bottom with parchment paper.
Cream together the butter and sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition.
In a separate bowl, whisk together the flour, cocoa powder, and baking 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 20-25 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely.
Melt the dark chocolate in a heatproof bowl set over a pan of simmering water (or in the microwave). Stir in the heavy cream until smooth and glossy.
Dissolve the gelatin in a small amount of warm water. Add the dissolved gelatin to the chocolate cream and mix well to combine.
Pour the chocolate mousse over the cooled cake layer. Refrigerate the cake for at least 3 hours, or preferably overnight, to allow the mousse to set completely.
Before serving, dust the top of the cake with cocoa powder, grated chocolate, or chocolate decorations.
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