Adjust the oven rack to the lower third of the oven; preheat the oven to 400°. Grease each popover cup generously with softened butter or solid vegetable shortening.
Combine the eggs, milk, melted butter, flour and salt in a blender or food processor; process for about 40 seconds. Be sure to scrape down the sides of the bowl a couple of times and blend until the mixture is completely smooth.
To make by hand, whisk together the eggs, milk and butter in a large bowl. Add the flour and salt and beat until very smooth.
Pour the batter into the greased cups, filling them half to two-thirds full. Bake until the popovers are puffed and golden brown, 35-40 minutes. Do not open the oven door during baking or the popovers will collapse.