This seems like it should be a simple task, but it’s actually a little difficult if you don’t know the right technique!  The best thing to do is to put a short drinking glass on your counter. Open a sandwich bag and put it inside the glass, but fold the edges of the bag around the edge of the glass so that the glass is holding the bag open.  Then use a rubber spatula to drop dollops of frosting into the bag while the cup holds it open. Once a sufficient amount of frosting is in the bag, unfold the edges from around the cup and twist the bag closed.  Cut off a tiny corner from the bag and squeeze the frosting out onto the Halloween brownies.