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.
The key to what you're trying to do is to not replace brown sugar, but to make it. Mix white sugar and treacle (or molasses, same thing) in a food processor. Start with about a tablespoon of treacle to 1 cup (200 grams) of white, granulated sugar. You might end wanting two or three times that amount of treacle. Just gauge it by color. That's how brown sugar is made.
EDIT I missed that you're using muscovado, that's fine. I lowballed it to recommend 1 TBS of treacle, so that's still in the ballpark.