Preheat oven to 350˚F. Butter and flour a 9-inch round cake pan.
Start by making the cake. Melt the butter and chocolate in a large heatproof bowl set over a pot of simmering water. Stir frequently until melted and smooth. Remove from the heat and set aside.
In another large bowl, whisk together the eggs and sugar. Slowly stir in the melted chocolate and butter until evenly combined. Add the flour and hazelnut powder and stir until incorporated. Transfer to your prepared pan and bake in the oven for 25 minutes or until a toothpick inserted comes out clean. Leave to cool for 10 minutes and then turn out onto a wire rack to cool completely.
Once the cake has cooled, make your ganache. Heat the cream in a pot over medium heat. You just want it to get hot, not simmer or boil. Once the cream is hot, remove from the heat and stir in the finely chopped chocolate. The finer the chocolate, the easier it will melt. Continue to stir the mixture until it is silky smooth. Leave to cool for 5-10 minutes so it thickens up and is not too runny when you spread it on your cake.
Place your cooled cake on your cake dish or pedestal and spread the ganache over top.
For the decorations, I like to use some toasted hazelnuts and Ferrero rocher chocolate balls. Finish with a sprinkle of chocolate shavings. A dessert as beautiful to look at as it is to eat!