For the batter, whisk the flour, salt, paprika, and cayenne pepper in a medium-size mixing bowl. Whisk the milk, eggs, and melted butter in a small mixing bowl. Pour the milk and egg mixture over the dry ingredients and combine to form a batter, using a whisk (this will be accomplished in about 15 strokes). The batter will be slightly lumpy. Refrigerate the batter, covered with a sheet of food-safe plastic wrap, for 30 minutes (or up to 3 hours).
Preheat the oven to 400 degrees F in advance of baking.
Film the bottom only of 6 large popover cups (6 cups to a pan, each cup measuring 3 inches in diameter [interior diameter, minus the lip; 3 3/4 inches in diameter if including the lip] and 2 1/2 inches deep, with a capacity of 2/3 cup) with nonstick oil spray made for high-heat cooking. Ten minutes before baking, place the prepared popover cups in the preheated oven.
To assemble the popovers, sprinkle the Parmesan and Gruyère over the batter and mix to combine, using a flexible rubber spatula.
Quickly and carefully, remove the pan from the oven. Divide the batter evenly among the prepared popover cups.
Place the popovers in the preheated oven, immediately raise the oven temperature to 425 degrees F, and bake for 30 minutes, or until risen, set, and deeply golden. The centers will remain slightly custardy. Resist the temptation to open the oven before 20 minutes, or the rise of the popovers will be compromised. Let the popovers stand in the pan on a cooling rack for 1 minute, remove them to a breadbasket or platter, and serve.
Recipe courtesy of Baking Style: Art, Craft, Recipes by Lisa Yockelson/Wiley, 2012.