"I don't think it is recommended to heat aspartane which is the sweetner
in diet Coke. I would try it with Diet Rite Cola which is sweetened with
Splenda- which you can heat."

Personally, I think I'd go with Regular Coke. I mean, you're spreading
the calories from a can over, what, about 24 brownies - it wouldn't add
a whole lot of them to each one. But I agree - if you're set on using
diet, go with Diet Rite or else I believe Coke has a new product which
is half sugar, half Splenda.