Everything in moderation, right? We eat other sugar filled foods for breakfast from time to time, so it doesn't seem that crazy. Though I'm hopeful you don't mean you're planning to make this an everyday thing!
If it feels good, do it! If it tastes good, eat it! I have never really cared for traditional breakfast foods like bacon and eggs, and my mother used to really get her feathers ruffled when I ate cake for breakfast. Now I go through the same argument with my husband. The man who loves cold pizza for breakfast thinks I should be satisfied with bacon and eggs..
Think about it. Just a few of the benefits: eggs build muscle strength and help prevent muscle loss, milk has needed vitamin D and protein, flour contains vitamins and minerals, and chocolate is a good source of magnesium. What is decadent about "good for you"? Viva chocolate cake for breakfast!
Chocolate cake wouldn't be a healthy everyday choice for breakfast, but I know a lot of people who eat donuts for breakfast sometimes and at least chocolate cake isn't fried! I recently made Moist Chocolate Cupcakes with Buttercream Frosting and I must admit, I ate one for breakfast the following day. They were even better the second day and so delicious with coffee. Today I almost followed a guy on Twitter until I read his profile. He said he was "morally opposed to cupcakes". I knew we weren't on the same wave length after that! If you love chocolate cake, I say go for it!!
Chocolate is okay anytime. Seriously, haven't you heard Bill Cosby's comic review about 'Chocolate Cake for breakfast?' If not, it's a must. It is hilarious. As he puts it: it has the basic food groups: flour, butter, eggs...