Preheat the oven to 180 C / Gas 4 and grease a 23cm square cake tin.
Cream the butter till light and fluffy. Whisk in the caster sugar, then add the eggs one at a time, beating between each addition. Mix in the salt.
Sift in the plain flour and baking powder and mix till smooth. Add the ground hazelnuts, grated chocolate and cinnamon and gently fold together till just combined. The mixture will be quite thick. Tip cake mixture into prepared tin and level the top.
Bake cake in the preheated oven for 50 minutes, or till a skewer inserted in the centre comes out clean. Let cool in the tin slightly, then turn out onto a serving plate and dust with icing sugar.