David, an 8.5 lbs Diamond would need around 1 ounce of dark chocolate to be concern about toxicity. In cake the amount of chocolate is usually less. So, if she ate less than 1 ounce of cake it is not likely for this to be systemically toxic. To be on the safe side you can go ahead and induce vomiting with peroxide. This is what you can do:
1- To induce vomiting give her 1 (one) teaspoon of hydrogen peroxide orally.
2- Allow her to walk for a minute and move her belly to mix stomach
content with peroxide. If she does not vomit within 5-10 minutes, repeat it again. Can give a total of 3 times 5-10 minutes apart.
3- If she does not vomit after 3rd dose or you do not see the chocolate in the vomit, observe her for the next 24 hours. If she becomes hyperactive, muscle
tremors, changes in respiration, etc. have her checked immediately by ER veterinarian, these are symptoms that could progress to life threatening seizures
or cardiac arrhythmias. If no symptoms after 24 hours (especially the first 12 hours), she will be fine. My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back. I am happy to address follow-up questions. Thank you for your business and I hope to work with you again soon!