First of all, splenda is formulated to sub 1 for 1 with granulated sugar. Granulated sugar weighs a little less than twice as much as icing sugar for the same amount of volume. With this in mind, you'll want to use about 2 1/2 C. splenda for this recipe.

Secondly, the recipe relies on the textural properties of sugar to create gooeyness/chewiness in the fudge, so texturally, you're going to be looking at a different animal.

Lastly, splenda, by itself, combined with chocolate seems to be problematic for quite a few people. The common complaint is that the final product isn't sweet enough, regardless of the amount of splenda added. This doesn't happen to everyone. I'm also not sure what causes it. My top theory has to do with the quantity of sweetener involved. The bitterness of chocolate requires a substantial amount of additional sweetener as compared to other desserts. Although splenda works wonderfully in small amounts, when you get to the amount required to sweeten chocolate, the aftertaste becomes quite noticeable. For whatever the reason, you might want to be aware of the issue. Using more than one sweetener is a sure fire way of resolving it.