If you suspect your dog has a yeast infection, you should have a culture done of the area where you feel this problem is. The vet's cortisone injection is great for inflammation but will do nothing for an infection of any sort.
Also, ingestion chocolate cannot cause a yeast infection either. Yeast infections externally are often caused by either contact exposure to a fungus or moisture build up, with common areas of occurance being the armpits, between the toes, and inner thighs.
Best wishes with your dachsund,
Christine
Certified Veterinary Technician
Licensed veterinary technician (B.S. Mercy College), 10 yrs in animal medicine and training