Preheat the oven to 350 degrees F or 175 degrees C. Spray and lightly flour a 10 inch round Bundt pan and set aside.
In the bowl of a stand mixer, combine the butter, sugar and light brown sugar. Cream together until smooth.
Add in the eggs, vanilla and black treacle and mix until combined.
Add in the dry ingredients - flour, cinnamon, ginger, nutmeg and mixed spice. Mix thoroughly - scraping the sides and mixing again.
Add in the sour cream and milk and mix until combined.
Pour the batter into the prepared bundt pan.
Bake in the preheated oven for 50-55 minutes or until the centre is set and a toothpick comes out clean.
Allow to cool for about 10 minutes before inverting on a cooling rack and carefully shaking to remove the cake from the pan. Allow to cool completely before decorating.
For the white chocolate glaze, melt the white chocolate in a bowl over some hot water or in the microwave. Once melted, add the cream to a saucepan and bring to the boil. Pour the cream over the white chocolate and mix together well.
Once the cake is cooled, spread the glaze over the top and let it flow down the sides.
Slice and serve - you'll get 12-16 portions, depending on how large a piece of cake you want to give everyone!