Preheat the oven to 180 C / Gas 4. Grease a 23cm cake tin and line with baking parchment.
Toss the chopped fruit and nut chocolate bar in a little flour (this will help prevent the pieces sinking to the bottom). Set aside.
Combine the sugar and butter in a mixing bowl and use a hand blender to cream together, until lighter in colour.
Into a separate bowl, sift the flour and cocoa powder; set aside.
Add your eggs to the butter and sugar mixture one at a time, blending with hand blender after adding each egg. Add vanilla extract and mix well.
Gradually fold in the flour and cocoa into the butter mixture, adding a tablespoon at a time and folding gently, turning the bowl as you go (this helps to get air into the mixture to give the cake a good texture). Add in the floured chopped chocolate bar, folding gently without over mixing.
Bake in the preheated oven on the middle shelf for 45 minutes (please note-depending on your oven, the cake may need more or less time.) The cake is done when a cocktail stick inserted into the centre comes out clean. If there is still mixture on the stick but the outside looks done, wrap with foil and continue cooking, checking every 10 minutes. Remove from the oven and allow to cool in the tin for a few minutes before transferring to a cooling rack. Enjoy warm or allow to cool completely before serving. You can also top with a chocolate glaze (see footnote)
If you wish to add a topping to this cake, try melting 85g plain chocolate in a microwave and stir in 30g butter or margarine and 1 tablespoon golden syrup. Spread on the cake and sprinkle on chopped mixed nuts if desired.