Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
In a pan on a low heat melted butter, black treacle and golden syrup together. Once melted, remove from the heat
In a large bowl stir together the self raising flour, bicarbonate of soda, ground ginger, mixed spice, cinnamon and dark brown sugar
Pour the treacle mixture into the flour mixture and whisk well
Whisk the egg and the milk together in a small bowl or jug, then add to the mixture a bit at a time, stirring well between each addition
Divide the mixture between the tins and bake for 30 minutes or until a skewer comes out clean. Leave to cool fully
For the filling, mix the lemon curd with the diced ginger and spread on the bottom sponge
For the icing, beat together the ginger syrup, cream cheese, double cream and icing sugar until thick using an electric mixer. Add it to the cake on top of the lemon curd filling, then sandwich with the other half of the cake
Serve immediately and store leftovers in the fridge for 2-3 days