Preheat the oven to 350°F (175°C).

Butter a glass baking dish.

In a large mixing bowl, cream together the softened butter and dark brown sugar.

Add the beaten eggs and mix until well combined.

Add the grated sweet potatoes, milk, molasses, brandy, cinnamon, nutmeg, allspice, salt, and orange zest to the bowl.

Stir until all ingredients are thoroughly combined.

Pour the sweet potato mixture into the prepared baking dish.

Sprinkle additional brown sugar evenly over the top of the mixture.

Bake in the preheated oven for approximately 1 hour, or until the pone is set.

Let cool slightly before serving.