Place a medium ovenproof skillet, preferably cast iron, in center of oven and preheat oven to 450°. Let skillet heat at least 25 minutes total (prepare your toppings while you wait). Blend eggs in a blender on high speed until very frothy, about 1 minute. With motor running, gradually stream in milk, then 2 Tbsp. butter; blend another 30 seconds. Add flour, cornstarch, salt, and pepper and blend just to combine. (This batter doubles well if you want to scale up.) Carefully remove skillet from oven and swirl remaining 1 Tbsp. butter in skillet to coat. Immediately pour batter into skillet (butter will brown quickly, so have batter at the ready to prevent burning). Bake pancake until puffed and brown around edges, 20–25 minutes. It will begin to deflate as soon as it comes out. Top as desired.