Preheat the oven to 450°F, and place your popover pan on a baking sheet in your oven to warm up while you are mixing your batter.
In a medium sized mixing bowl, whisk together the eggs and milk until frothy, about 1 minute. Add the flour and salt to egg mixture and whisk until batter is the consistency of heavy cream with some small lumps remaining. You want to see some small air bubbles because they are what will cause the popovers to rise but don't over mix. Add in the cheddar cheese and chives and mix again lightly.
Remove your popover pan from the oven and grease or spray the muffin cups. Fill the cups 2/3 full (3/4 full for large popovers), and bake at 450 degrees for 20 minutes*. Then, reduce the oven temp to 350 degrees and bake for an additional 15 to 20 minutes. (Do not open the oven! Opening the oven door in the first 20 minutes can cause them to collapse.)
When they are brown and crusty, remove them from the oven, and make a small slit in their sides with a sharp knife. "Popovers lose their crunch if they linger in the pan, so turn them out on a wire rack immediately and poke a small opening in the side of each with a paring knife to let the steam escape." Serve immediately and enjoy!