Place a popover pan in the oven on a bottom shelf. Position a second shelf about 12 inches above the bottom shelf. Place a sheet pan on the top shelf (this will help protect the popovers from over-browning.
Preheat the oven to 450°F. Continue heating the oven for 20 minutes after the oven registers 450°F.
Meanwhile, place 2 tablespoons of melted butter, eggs and milk in a blender and pulse to blend completely. Add the flour and salt and pulse several more times until combined. Leave the mixture to set for 15 minutes.
When the oven has been thoroughly preheated, carefully pour a drizzle of melted butter (using the remaining 2 Tablespoons) into the bottom of each cup - the butter may splatter so be careful. Work quickly to fill each cup with a bit of the butter.
Pour the rested batter into each cup - filling about 2⁄3 of the way full.
Bake for 20 minutes, then turn down the oven to 350°F (without opening the oven!) and continue baking for 10 minutes more.
Remove the popovers from the oven and serve immediately with butter and/or jam, if you wish.
Notes
For the best results, follow the recommendations for room temperature and rested batter, preheating the oven and popover pan adequately, and pouring the melted butter into the popover tin. These three factors together make a big difference in the finished result.
Be sure to properly measure your flour to ensure a light and airy popover. To measure flour properly, use the scoop and sweep method. Aerate the flour with a spoon, then gently spoon the flour into a measuring cup until it's full. Level it with the flat end of a butter knife, but do not pack down the flour at all. You should never tap a measuring cup with flour onto the counter or other surface. If there is too much flour you will be left with dense popovers.
The popovers need to cook long enough for the steam to really puff the batter and then for the batter to form a tough enough shell that the popovers hold (most) of their shape. Because of this, the tops of the popovers can become too dark. Placing a sheet pan on the top rack does make a difference in blocking that top heat and keeps the popovers golden brown while having enough time to cook all the way.