I made the brownies Saturday night. I ended up making two boxes of mix, but I am pretty sure one would've been enough for almost 40 "burgers". I made them in a bigger cookie sheet and tried to spread them thin - you can spread them really thin and just reduce the baking time and they will be great! (Though it couldn't hurt to have a back-up box of mix around just in cases.)
I also made a test batch of little cookies to get the size right - and then found something round to cut out "burgers" to match the size of the cookies. I used the lid to our can of Pam, and it worked perfectly. I also sprinkled sesame seeds on half of the cookies for the top half of the "buns" - the sesame seed bun and the lettuce are my favorite parts. The lettuce is just shredded coconut. I put it in a bowl and put a few drops of green food coloring in and mixed, adding more food coloring as I went to get the green I wanted. To make the ketchup I just made some icing out of powdered sugar, a tiny bit of vanilla extract, and water, and then added red food coloring to make it ketchup. I kept it pretty thick so it wouldn't run all over the place.
Putting them together was easy - I put a small dot of "ketchup" in the middle of the bottom bun to hold the burger in place, layered on a burger, squeezed some ketchup around the edge of the burger (because it's really for looks more than taste, so I didn't want to put much), put some lettuce on top of that and then the sesame seed bun to finish it off.
You could also do plain coconut as onions, yellow icing as mustard, or try to make pickles out of jelly candies of some sort? These may not be what you have in mind when you're craving a burger, but they sure are cute!