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.