In a bowl, mix the butter at room temperature with a pinch of salt and sugar. Add eggs one at a time and mix after each one.
Separately in a bowl mix flour, cocoa, coffee, baking powder and baking soda. Gradually add the dry ingredients over the butter and egg mixture.
Melt the chocolate on a steam bath and incorporate it at the end, mixing again with the mixer on step 1.
Grease a tray with a little oil and line with baking paper.
Pour the composition into the pan, level with a spatula and put the pan in the preheated oven at 170 degrees for 35 minutes, the last 15 minutes reducing the temperature to 160 degrees.
Remove the tray from the oven and after it has cooled, take out the cake on a plate.
Melt the chocolate together with the butter on a steam bath and grease the cake with this icing.
Sprinkle chopped hazelnuts over the icing (you can also fry them beforehand).
The cake can be sliced and served immediately after the icing has hardened.