Place a medium ovenproof skillet, preferably cast iron, in the center of the oven and preheat the oven to 450°. Let the skillet heat for 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 the oven and swirl the remaining 1 Tbsp. butter in the skillet to coat. Immediately pour the batter into the skillet (the butter will brown quickly, so have the batter ready to prevent burning).
Bake the pancake until it is puffed up and brown around the edges, 20–25 minutes. It will begin to deflate as soon as it comes out. Top as desired.