Butter: In a small bowl, mix together the butter, anchovy paste, chives, lemon juice, and lemon zest until smooth. Season with salt and pepper (if using), to taste.
Popovers: Put an oven rack in the center of the oven. Preheat the oven to 400 degrees F. Spray a 12-count muffin or popover pan with vegetable oil cooking spray. Set aside.
In a blender, add the eggs and garlic. Blend on medium speed until frothy, about 15 seconds. Add the milk and the cheeses and blend until smooth. Add the flour, salt, pepper and parsley. Blend on medium speed until incorporated. Pour the batter into the muffin cups, filling each cup to within 1/4-inch of the top. Bake, without opening the oven door, until puffed and golden, about 30 to 35 minutes. Remove the popovers from the oven and serve with the anchovy butter.