After some trials....here are my tips:
Pre-heat oven - 325 degrees
Approximately 26 minutes (it is probably longer than that, but just keep checking)
If you add marshmallows directly in the batter, make sure you leave like 5 mm of space in the dish (don't fill the shape all the way) OR try taking the brownies out after 20 minutes and put the marshmallows on top of the brownies then put the brownies back in the oven.
Hopefully I'll get a chance to put a picture of the brownies up soon!
Hope you've already made it over to Mommy & Me Creations! If not check out all the Valentine's Day ideas:)