Preheat the oven to 425°. In a large bowl, whisk the eggs with the sugar and orange zest. Whisk in the milk and 3 tablespoons of the melted butter. In another bowl, whisk the flour with the baking powder and salt. Whisk the wet ingredients into the dry ingredients until only small lumps remain.
Brush the cups of a muffin tin (preferably not nonstick) with the remaining 1 tablespoon of melted butter and heat the muffin tin in the oven for 5 minutes; the butter will turn a nutty brown. Carefully fill the muffin cups halfway with the popover batter. Bake the popovers for about 30 minutes, until they are risen and browned. Turn the popovers out onto a serving platter and serve them right away.