While you can substitute real marshmallows for the marshmallow creme, they will definitely have a different texture. The marshmallow creme and chocolate frosting combine and swirl in a way that marshmallows won’t. If you don’t have marshmallow creme and want to use marshmallows, I recommend using mini marshmallows and putting them on top of the cake for its last 2-3 minutes in the oven. That way, they’ll melt a little and combine better with the frosting.