1. In a large pot, heat oil and 2 test kernels with lid closed over medium-high heat. Once those first kernels pop, add rest of kernels and cook until popping stops. (Shake constantly.)
2. In a large mixing bowl, drizzle popcorn with melted butter a little bit at a time, mixing in between butter additions to avoid hot spots.
3. Shake hot cocoa mix over butter popcorn one tablespoon at a time, mixing/tossing in between cocoa additions.
4. Salt to taste, mix in marshmallows, and serve!