Although adding the mincemeat gives both an extra dimension and flavour, feel free to leave it out and rely instead on the naked cake itself to create the Christmas pudding appearance.
A range of possible reconciliations have been proposed, including leaky layering at 660 km, layering deeper in the mantle, or ubiquitous compositional heterogeneity like a 'plum pudding'.
Use cling film to neatly line a 1.2 litre/2 pint pudding bowl, leaving extra cling ... (Put them in a plastic bag first so they don't go everywhere!) Melt the chocolate, butter and golden syrup ...