Start by lining a 23cm round cake tin with non-stick baking paper. Next, beat the butter into the melted chocolate until all the butter is incorporated. Put aside. In another bowl, whisk together the egg yolks and the sugar until pale, light and fluffy using an electric whisk. Gently fold in the chocolate and butter mixture using a large spoon, followed by the ground hazelnuts and the flour. Finally, fold in the orange zest and liqueur. Finally, whisk the egg whites with the salt in another bowl until stiff peaks form, then carefully fold into the cake mixture, trying to keep in as much air as possible. Pour the cake mixture into the prepared tin and bake at 190`c for 30-40 minutes, until firm to the touch. (It will still be moist in the centre so the usual skewer test won’t work.) When the cake is cooked, remove from the oven and allow to cool for 5-10 minutes before turning out onto a cake rack to finish cooling. Meanwhile, you can make the glaze by placing the chocolate into a bowl set over a pan of gently simmering water and allow it to slowly melt. Just before all the chocolate is melted, remove the bowl from the heat and beat in the butter bit by bit with a wooden spoon and add the liqueur if using. Leave the glaze to cool for 5-10 minutes, until it begins to thicken, but is still runny. Spread the glaze over the cake and down the sides, using a palette knife. Decorate the top with the chopped hazelnuts before the glaze sets.