Tear the sourdough loaf into small pieces and place them in a bowl.
Pour the milk over the bread pieces in the bowl.
Add the dark brown sugar, eggs, sultanas, and mixed spice to the bowl.
Mix everything together, ensuring the bread absorbs the milk and egg mixture.
Allow the mixture to sit for about 20 to 30 minutes to soak.
Melt the butter and incorporate it into the mixture thoroughly.
Transfer the mixture into a nine-inch baking tin, leveling it out and pressing it down slightly.
Sprinkle demerara sugar on top to create a sweet crust.
Bake in the oven at 140 degrees Celsius for approximately 45 minutes.
