Preheat the oven to 170C(306F) fan-forced and line a loaf tin with baking paper. Make sure you extend the paper over the sides so you have little handles to remove it from the tin.

Use and electric mixer to beat the butter, syrup, sugar and melted chocolate until smooth.

Mix the flour, bicarb and cocoa together. With the motor running, add 1 tablespoon of the flour mixture to the bowl followed by one egg. When that has been mixed in add another tablespoon of flour and the remaining egg then add the rest of the dry ingredients.

Keep the beater going and add the orange and orange zest and mix until just incorporated. Nigella warns that the batter may curdle a little at this stage but it's nothing to worry about. I'm glad she added that note because mine curdled and I would have been a little concerned.

Bake a single loaf cake for 45 minutes or baby loaf cakes for 20-25 minutes but check both about 5 minutes before they should be ready. When it springs back in the middle when you touch it lightly it's ready to come out.

Leave the loaf in the tin for about 5 minutes before turning out onto a wire rack to cool completely.