I understand that you are concerned that your pup ate part of a Betty Crocker dark chocolate cookie. Chocolate is toxic, but they do need to eat a certain amount per pound of body weight to get sick.
At 50 pounds she would need to eat 3 ounces of dark chocolate to see symptoms of drooling, nausea, vomiting and diarrhea and 6.5 ounces of dark chocolate to see hyperexcitability and an elevated heart rate.
At higher amounts we can see muscle tremors and possible seizures.
To give you a point of reference most normal chocolate bars are 2.25 ounces. One cookie is very unlikely to have enough ounces of dark chocolate to make a 50 pound dog sick.
There are things you can do to lessen signs of chocolate toxicity asa precaution.
I would encourage you to get her to drink as much as possible and to get her out to urinate frequently. Chocolate toxins are metabolized out through the kidneys and can be reabsorbed from the bladder so we don't want them to sit in her bladder for long.
In most cases mild chocolate toxicity is out of their system in 24 to 48 hours. We generally begin to see symptoms, if they eat a toxic amount, within 4 to 6 hours.
If she seems at all nauseous it may help to with-hold food for the next 12 hours to stop intestinal spasms.
To try and settle her stomach you can give either:
1) Pepcid ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours
These are acid reducers and should help her feel less nauseous. They are quite safe and can be used for several days if needed.
Then further to decrease the symptoms of stomach upset you can feed her a bland diet for the next couple of days. A homemade bland diet is a mix of 1/3 boiled, minced, white skinless chicken or lean boiled hamburger and 2/3 boiled, white rice mixed with some low salt chicken broth to make it easy to lap up and swallow. Feed small meals frequently.
If you do notice a very rapid heart rate (more than 160 to 180 beats per minute in a small dog) repetitive tremors, or seizures then she should be seen by a veterinarian on an emergency basis for treatment. They can use injectable medications to control her heart rate and stop the seizures until the toxic levels fall.
But truly one cookie is not likely to cause any toxicity symptoms in a 50 pound dog.
Best of luck with your girl, let me know if you have any further questions.