Preheat oven & cast iron skillet: Place a 10-inch or 12-inch heavy cast-iron skillet on the center rack of your oven and preheat to 425°F (220°C). The skillet must get intensely hot to create the dramatic puffy rise.

Blend the batter: In a blender or food processor, combine eggs, warm milk, flour, granulated sugar, vanilla, cinnamon, and sea salt. Blend on high speed for 30 to 45 seconds until frothy and lump-free. Let the batter rest on the counter for 10 minutes to allow the flour to hydrate.

Sauté cinnamon apples: While the batter rests, melt 1 tablespoon of butter in a saucepan over medium heat. Add sliced apples, brown sugar, cinnamon, nutmeg, and lemon juice. Cook for 4 to 5 minutes until apples are slightly tender and glazed in a fragrant spiced syrup. Remove from heat.

Sizzle butter in skillet: Carefully remove the scorching-hot cast-iron skillet from the oven using heavy-duty oven mitts. Drop the remaining 2 tablespoons of butter into the hot pan and swirl rapidly to coat the bottom and sides in foaming, melted golden butter.

Pour & bake: Immediately pour the rested blender batter into the center of the sizzling skillet. Spoon half of the warm sautéed apples into the middle. Return the skillet to the 425°F oven immediately.

Bake undisturbed: Bake for 18 to 20 minutes without opening the oven door. The pancake will climb high up the sides of the skillet, puffing into dramatic golden wavy ridges with crisp browned edges.

Garnish & serve: Carefully pull the skillet from the oven (it will naturally settle slightly in the center). Spoon the remaining warm caramelized apples and spiced syrup into the center crater, shower generously with powdered confectioners sugar, and serve warm with pure maple syrup.