Place the oven rack in the bottom third of the oven and preheat the oven to 450°F.
Prepare the popover pans or muffin tins (with enough wells to make 16 popovers) by placing a dot of butter in the bottom of each of the 16 wells. Heat the pans or tins in the oven while you make the popover batter.
Warm the milk in a small saucepan over medium heat. It should be hot, but do not bring it to a boil. Remove from the heat.
In a large bowl, whisk the eggs with the salt and black pepper until smooth. Stir in the reserved warm milk.
Add the flour to the egg mixture and combine. The batter should have the consistency of cream. A few lumps are okay!
Remove the popover pans or muffin tins from the oven.
Spray the 16 wells generously with nonstick cooking spray.
Pour about ½ cup of the batter into each well.
Place several Gruyère and Black Pepper Popovers cubes of cheese on top of the batter in each well.
Reduce the oven temperature to 350°F.
Bake the popovers until the tops puff up and are golden brown, about 40 minutes. Remember not to open the oven door while baking. You don't want the popovers to collapse!
Remove the popovers from the oven and turn them onto a wire cooling rack right away to preserve their crispy edges.
Using a sharp knife, pierce the base of each popover to release the steam. Sprinkle grated Gruyère over the finished popovers, if desired, and serve immediately.