substituting black treacle/molasses for brown sugar I would like to replace some of the brown sugar in a dark fruit cake recipe with black treacle (similar to molasses I believe), as I like the rich taste and think it will add to the moistness of the cake. But I don't know what quantity to use instead. The recipe calls for 200 grams of brown sugar, so I was thinking of replacing half of it maybe? I've seen a different recipe for a similar size cake that uses 150 g brown sugar and 1 tablespoon of black treacle, so perhaps that would be a better idea? I'm using the dark soft muscovado sugar as the recipe suggests, but I really want a dark, rich, moist, almost sticky cake, almost more like a Christmas pudding in texture. Hope someone can help :-)
A standard substitute for brown sugar is to take 1 Cup = 200 g of white granulated sugar, and mix-in 1 tablespoon of molasses/treacle.  There's a lot of flavor in that tablespoon.  

My own recommendation would be to either supplement your full 200 g of brown sugar with a tablespoon of treacle or consider using 200 g of granulated sugar and mix-in 2 tablespoons of treacle.

Since you are planning to use dark soft muscovado sugar (which is already flavorful) in your recipe, I think supplementing the full 200 g amount with 1 tablespoon of treacle would give your cake a terrific taste.