Heat the oil in a large lidded saucepan over medium-high heat.

Add a few corn kernels and cover. When the kernels start popping, add the remaining corn, cover, and remove from the heat. Allow to rest in the pot for 30 seconds (you may hear some popping during this time), then return to medium heat. Continue to cook with the lid slightly ajar, shaking the pan occasionally, until the popping begins to slow down (there will be a few second intervals between the pops), about 1-2 minutes. Remove from the heat and add to a bowl.

Add the butter, salt, garam masala, and cayenne to the pot. Swirl until the butter is melted. Add the popcorn back to the pot and toss until fully coated in the butter and spices. Taste for seasoning and add more salt or spice as necessary. Serve immediately.