*Tips For Cutting Marshmallow Brownies. These brownies have a gooey layer of marshmallow in the middle and a thin chocolate frosting on top. They will cut best when chilled or even frozen. I use a long, sharp knife to make straight down cuts (no sawing motion!) and clean the knife off in between slices. At minimum, make sure the brownies are cooled COMPLETELY before slicing, or they'll rip and tear.