Mix flour and salt in a medium bowl. Mix milk, eggs, and butter (or beef drippings, if using) in a 1-quart Pyrex measuring cup. Pour egg mixture into flour mixture; stir until smooth. Return mixture to the measuring cup for easy pouring.
Spray a muffin tin that has 12 individual cups (each with 1/2-cup capacity) with vegetable cooking spray. Fill each cup 3/4 full. Adjust oven rack to lower middle position and set muffin tin on rack. Close oven door and heat oven to 425 degrees. Bake, without opening the oven door, until popovers are crisp and golden brown, 35 minutes. Remove from oven and transfer to a wire rack. Recrisp in a warm oven before serving.