Prepare a 12-cup popover pan by placing ½ teaspoon of butter in the bottom of each cup.
Preheat oven to 400°F with the 12-cup popover pan in the bottom third of an oven.
In a saucepan over medium high heat warm the milk until you begin to see small bubbles around the edges.
Combine the flour and salt and run it through a sieve to make it extra fine. Set aside.
In a large bowl whisk the eggs until they are frothy.
Remove the milk from the heat and slowly pour it into the egg mixture, whisking continuously. Be careful that you don't go to fast and cook the eggs!
Once the milk and eggs are combined add the flour mixture, whisking it until there are no lumps and the mixture is smooth.
Remove the popover pan from the oven and fill each cup 3/4 full of batter.
Top the batter in each cup with 1/2 ounce of Gruyere cheese.
Place the pan back in the oven with a baking sheet beneath it on the rack below to catch any drips, and bake for 45 minutes. Do not open the oven during this time.
Once the popovers are done remove them from the oven, cut a slit in the bottom of each popover to release the steam, and place them in a bread basket. Serve immediately.