It's very difficult to gauge the calories of chocolate cakes. It depends on what is used, and the type of chocolate cake.. Chocolate cakes are my all time favourite. So far the best chocolate cake I have tasted is the Devil's Chocolate Cake in Epicurious. Love it!
Calories of 1 slice of chocolate cake is around 400-700kcal.
How I keep the calories in check? Well, I have a rather pathetic way. I bought my slice of cake, keep it in the fridge and took only a mouthful each day. Sad right? :(