Add the shredded white cheddar and chopped chives, and mix with a spoon or spatula. Pour the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake the Popovers at 450°F for 20 minutes, then reduce the heat to 350°F and continue baking for another 10 minutes, or until the Popovers are golden brown and puffed.