Preheat the oven to 350°F (180°C). Grease and line a 7-inch cake pan.
Melt the dark chocolate, butter, and milk for the brownie base; combine.
In another bowl, mix egg, salt, sugar, and vanilla.
Sift and mix in flour, cocoa powder, baking powder, and hazelnuts.
Bake in the prepared pan for 20 minutes. Allow to cool, then trim to fit a 6-inch ring.
Soak gelatine. Whip cream and powdered sugar to peaks.
For each chocolate mousse layer, melt chocolate with some heavy cream. Combine it with the whipped cream once cooled. Layer over the base and chill for 20 minutes.
Once all layers are added, chill the cake for at least 5 hours.
Smooth sides and dust with cocoa before serving.