Set the oven to 180C/350F/Gas 4. Use baking parchment to line a 24cm/9½in square cake tin or baking dish.
Sift the flour, baking powder, bicarbonate of soda, cinnamon and ginger into a bowl.
Warm the golden syrup, honey and butter very gently in a pan until the butter and sugar melts. Stir in the muscovardo sugar. Set aside and allow to cool slightly.
Halve the plums, or cut them into quarters if they are very large, and remove the stones.
Break the eggs into a bowl, pour in the milk and whisk lightly to mix.
Pour the golden syrup mixture into the flour and mix with a large, metal spoon. Pour in the egg and milk and continue stirring till you have a loose, almost sloppy batter without any traces of flour.
Tip the mixture into the lined cake tin, drop in the plums and bake for 30-35 minutes.
Now for the topping; halve and de-stone the plums and add to the same pan as you cooked the syrup and honey. Add the sugar, butter and the syrup from a jar of crystallised ginger. Cover and leave to cook down.
Serve the cake with a generous topping of the spicy plums and a dollop of cream.