I've started using the microwave instead of the stovetop for brownies — I melt the butter in a large pyrex measuring cup, then add the sugar and microwave another 20 seconds. From there, I proceed as the recipe says!
If you want, you can add half of the marshmallows to the batter as well. I found that they melted and disappeared, so it really wasn't noticeable, so I didn't include that as part of the recipe.