I did a popcorn bag cake (in my pix) and used real popcorn coated in white chocolate for the popcorn. It's a fondant cake but very easy to make. Is this the kind of cake you're asking about?
I've seen these done before and typically you top your cupcake with some buttercream then take mini marshmellows that you have quartered the tops of ( by that I mean you snip the top of the marshmallow with scissors on way then again the other way, so it is split into fours on top) Then you strategically place those marshmallows on the cupcake until it looks like a heaping bucket of popcorn. Lastly you can add yellow sugar on top as kind of a buttered popcorn look. Also there are striped cupcakes liners that really add to the popcorn effect. I like to get mine at fancyflour.com
Hope that helps.
Sorry, I read the title too fast and didn't realize you were asking about cupcakes, not cakes. 
When I made mine, I iced with butter cream first. Took some mini marshmallows snipped with scissors across the tops in an "X" put on a paper plate, sprayed them with some yellow wilton spray, (didn't want to break out the airbrush for 1 doz cuppies). then put on top of the bc.
Simple simple. have fun!!! 
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