How do I incorporate chiles into a chocolate cake recipe?

An easy way of incorporating a spicy kick into any existing chocolate cake recipe that uses actual chocolate would be to substitute a portion of the called for chocolate (with a matching chocolate type; i.e. dark and dark, milk and milk) that already has cayenne or some other hot pepper ingredient. Of course, you will need to experiment to get the right level of heat.

Adding straight cayenne powder while the chocolate is melted, or shredded would also be an ideal time to ensure even distribution of the spice. In the event your recipe does not use an actual chocolate base, but rather something that replicates the flavor, the time to add the cayenne would be when the 'flavor packet' is added.

Have you tried to directly use spicy chocolate? I used chili aromatized chocolate and also black-pepper chocolate in some recipes with very good results.

My approach would be to choose a specific variety of chili pepper to give it a more localized flavor that goes well with the chocolate and the rest of the dessert. For example, habaneros are fruity, moritas smoky and so forth. Two ways to go about it:

1. Grind the pepper (or buy it preground) and add to the dry ingredients.
2. Infuse a dried whole or halved and seeded pepper into the liquid ingredients. This will be most effective if there is a melting step involved, like melting chocolate or butter. You would just let it sit in the warm liquid, stirring occasionally, until you found the desired degree of heat.

I haven't done this, but I'd try pureeing fresh peppers, then cooking in a small non-stick pan to remove excess water until it is the consistency of jam. Let it cool and add a few spoons to the batter. If adding 1/4 cup of cooked pepper puree, I'd probably decrease the fat in the recipe by 1 tablespoon, and decrease the liquid by 1 tablespoon.

I would start with a chocolate cupcake recipe that calls for a ganache as a filling or frosting or use a chocolate cake that has a ganache frosting or uses ganache between layers. Then I would make a chili chocolate ganage by using ground dried peppers (anchos, for example) and perhaps some cayenne pepper.

There are obviously other ways, but I think this would be an excellent application.