To make the ganache, put the chopped chocolate in a mixing bowl. Put the orange juice in the microwave and bring it to the boil. Put the cream in a separate pan and bring to the boil.
Pour the cream over the chopped chocolate and mix until it reaches a smooth consistency. Mix in the orange juice and the softened butter until smooth.
Leave on the side until required.
Heat the oven to 170C (Fan), 325F/Gas Mark 3. Line a 20cm square cake tin with foil.
In a saucepan, melt the ganache with the butter, then set aside somewhere warm.
Beat the eggs with the sugars until the sugar has almost dissolved and the mixture has turned creamy and a light beige colour.
Next beat in the chocolate ganache and butter until evenly combined, then beat in the orange juice.
Sift the flour, cocoa and mixed spice and then beat into the chocolate mixture.
Scrape the mixture into the prepared cake tin and bake for 25 minutes, or until a toothpick poked through the sugary crust comes through barely clean - the hot chocolate will carry on cooking for at least five minutes after you've taken it out of the oven.
Let the cake cool and spread the top with the remaining ganache and sprinkle the cacao nibs on top.
When the ganache has set, cut the cake into portions.
