My 8 pound dog ate half my dark chocolate brownie whe I got up to get some milk is she going to be ok ?

Probably. What you need to figure out is what the "lethal dose" ("LD50") is for chocolate for a dog. This is usually measured in grams or ounces of what ever the possible toxin is to grams, kilograms, etc of the animal's weight (i learned this because my daughter's dog recently ate a couple of ant poison stakes). So if you google something like "LD chocolate dog" or "LD50 chocolate dog" there should be something that will tell you how much your dog can eat before he has symptoms and how much he can eat before he is at risk of dying. Just remember that it's the weight of the actual chocolate that counts, not the whole brownie (so if the brownie weighs 10oz, but most of it is the flour etc. you have to guess how much chocolate is in the actual brownie).