Divide butter among popover cups, place in oven, and set oven at 450.

Whisk eggs, oil, and milk in a medium sized bowl.

Add flour and salt until just blended.

Remove popover pan from oven and spray with cooking spray, then pour batter unto cups.

Bake 20 minutes on lower oven rack.

Reduce heat to 350. Turn pan and place on upper rack. bake 7-12 minutes more, until brown and crispy.

Remove from pan and pierce each popover with a sharp knife to vent.