Marshmallows are basically a soft candy confection composed entirely of sugar, water, and gelatin whipped into an airy foam that is then cooled and set in a shape and coated with some corn starch to give a non-sticky coating. When a marshmallow is toasted over a flame, the outer portion is caramelized, meaning the sugar molecules burn to produce elemental carbon, hence the black color. The actual brand, I would think, would have little affect on the toasting time since they are all pretty much made the same way. The size of the marshmallow would have a much greater affect since the ratio of the surface area to total volume would be different for different sizes and thus would be exposed to heat differently. But as for actual manufacturing brand, I wouldn't think so.