When use use sugar in a recipe is adds moisture. When
you heat sugar it turns to a liquid and splenda does not. I don't use splenda but people who do use it say there is a problem with dry baked goods.
I am not sure how much extra liquid you need to add (per cup of splenda)but I would hope that Splenda has a website with that information.
I wonder if anyone knows how much liquid is created when you melt 1 cup of sugar to make caramel. I would add approximately that much liquid. Either water or milk, what ever liquid, if any, that your recipe calls for. The suggestion for using applesauce is a good one too.