Whisk the sugar, oil and eggs together then mix in the carrots and dates.
Crush the hazelnuts by pushing down on them, with your palm onto the flat side of large knife. If you use a food processor instead, you just want them broken, not powdered as they add a wonderful texture to the cake.
Add the nuts, flour, bicarb, cinnamon and orange zest to the egg mixture and mix together gently, until all the dry ingredients are incorporated.
Scrape into a greased & lined 7"x7" baking tin, flatten with the back of a spoon and bake for 30-35 minutes until firm and a skewer inserted into the middle comes out clean.
Lift out of the tin and let cool on a cooling rack.
In a food processor (you can do it by hand but do soften the butter first!) mix the butter until soft. Slowly add the other ingredients and continue mixing until blended well.