Heat the oven to 425°F. Remove all of the racks from the oven except one arranged in the lower third position. Place a baking sheet in the oven while it heats. Place 1/2 teaspoon of the butter into each well of a 12-well muffin pan; set aside. Place the remaining 1 tablespoon butter, eggs, milk, sugar, and salt in a blender and blend until smooth, about 30 seconds. Turn off the blender, add the flour, replace the lid, and blend until just smooth, about 30 seconds. Set the mixture aside in the blender. Place the prepared muffin pan in the oven on the heated baking sheet and bake until the butter sizzles, about 2 to 3 minutes. Remove the muffin pan and the baking sheet from the oven. Pouring from the blender, fill the wells of the muffin pan halfway up with batter. Return the muffin pan and baking sheet to the oven. Bake until the popovers are puffed and the tops are starting to brown, about 20 minutes. (Do not open the oven door during the baking time.) Reduce the oven temperature to 350°F and bake until the popovers are golden brown all over, about 15 minutes more. (Do not open the oven door until the popovers have baked at least 35 minutes.) Remove the muffin pan and baking sheet from the oven and place them on a wire rack. Remove the popovers from the pan and serve immediately.