In a small bowl, combine the gluten-free flour, tapioca starch, cornstarch, and kosher salt. Whisk together and set aside.

In a large mixing bowl, whisk the room-temperature eggs until light and airy. Slowly add the room-temperature milk, mixing until frothy. Gradually add the dry ingredients, stirring until a smooth batter forms.

Transfer the batter to a liquid measuring cup for easier pouring into the muffin pan. Let it rest at room temperature for 25-30 minutes while preheating the oven to 450°F, arranging the oven racks in the lower third.

Place the muffin pan on a sheet tray and heat it in the oven for 5-10 minutes. Carefully remove the tray, spray the muffin pan with non-stick spray, and fill the cavities with batter up to ⅔ full or slightly more.

Return the muffin pan to the oven and bake at 450°F for 15 minutes. Without opening the door, reduce the temperature to 400°F and bake for another 10 minutes. Then, reduce to 350°F and bake for an additional 5-10 minutes until dark golden brown. Do not open the oven during the entire baking period.

After around 30 minutes of total baking time, open the oven door and carefully poke each popover with a wooden skewer to release some steam. Close the oven door and bake for an additional 2-3 minutes. The popovers should be dark golden brown and tall at this point.

Remove the popovers from the oven and let them sit in the pan for a minute or two to help them set. Then, remove them from the pan and serve immediately.