Prepare Apricot Butter; set aside.
Preheat oven to 450°.
Combine egg whites and milk, stirring with a whisk until foamy. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, sugar, and salt, stirring well with a whisk.
Coat a 12-cup muffin pan with cooking spray, and place in a 450° oven for 3 minutes.
Gradually stir flour mixture into egg mixture, stirring with a whisk just until blended. Remove muffin pan from oven. Divide batter evenly among hot cups.
Bake at 450° for 15 minutes. Reduce heat to 350°. (Do not remove popover cups from oven.) Bake an additional 20 minutes or until golden.
Serve popovers immediately with Apricot Butter, if desired.