Once oil is hot add one or two kernels and check if it pops.
Once it pops add the rest of the kernels and close the lid.
It will start to crack and you will hear the cracking sound.
Keep shaking the pan in between so all kernels will pop.
Once you stop hearing the sound, switch off the flame and open the lid.
Heat butter in a pan and add salt.
Add this mixture to the popcorn and mix well
Butter popcorn is ready. Store in airtight container.
Notes
You can skip oil and use full butter to make popcorn. Store in airtight container and it keep good in room temperature for few days. You can add some masala to make it more flavorful. You can use any pan or pressure cooker to make this. Vessel should be wide so it is easy to pop.