Heat oil in a 2- to 3-quart saucepan or pot with a lid set over medium-high heat. Pour in popcorn kernels and sprinkle enough salt to cover the layer of kernels lightly. Add butter to the pot and cover with the lid.

As soon as kernels start to pop, shake the pot back and forth across the burner constantly until the popping slows down. When the pops are about 2 seconds apart, remove from heat and pour popcorn into a serving bowl.