Nancy's Pies has a sugar free chocolate cupcake line that is low carb - only 6g of carbs per cupcake. These are great for people who need sugar-free desserts!
We find these in the bakery area of our local supermarket. They're sold in sets of four, in the clear plastic 'box' that you often find bakery cookies or cupcakes in. They are a rich chocolate cupcake with a chocolate frosting.
The sweetener here is maltitol - so be sure not to eat all four at once unless you want a bit of intestinal discomfort later on! Your intestines aren't really meant to deal with large amounts of sugar alcohols :) Still, we haven't had any problems when we ate them in the proper amount of one per person.
I do have to comment that these are VERY crumbly. When I ate the first two cupcakes (at different times!) I ended up with cupcake crumbs all over creation. As we got through later batches, we got wiser. Usually we ate the cupcake in a bowl with some ice cream, which was just about perfect.
If you can find these, and you're in the mood for cake-and-ice cream, enjoy!