Put the oil and popping corn into a large saucepan. Cover with a tight-fitting lid and place the pan over a medium heat. Leave for 2–3 minutes until you hear the first pop. Shake the pan firmly, keeping the lid in place and the pan on the heat.
When the popping stops, remove from the heat and tip the popcorn into a bowl. Add the butter to the pan with the chocolate powder and heat to melt. Pour over the popcorn and stir well.
Add the marshmallows and serve in paper cones or cups.