Preheat oven and 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). Heating the skillet thoroughly is the golden rule for achieving an incredible dramatic puff.
Blend the batter: In a blender or food processor, combine room temperature eggs, whole milk, flour, granulated sugar, vanilla extract, and sea salt. Blend on high speed for 30 to 45 seconds until completely smooth and frothy with air bubbles. Let the batter rest at room temperature while preparing the apples.
Caramelize the apples: In a separate medium saucepan over medium heat, melt 2 tablespoons of butter. Add the sliced apples, brown sugar, cinnamon, nutmeg, and lemon juice. Sauté for 5 to 6 minutes, stirring frequently, until the apples are tender and glazed in amber spiced syrup. Remove from heat.
Melt butter in the smoking hot skillet: Carefully remove the scorching hot cast-iron skillet from the oven using heavy-duty oven mitts. Drop 3 tablespoons of butter directly into the pan, swirling rapidly until melted and foaming up the sides.
Pour and bake: Immediately scatter half of the warm spiced apples across the bottom of the sizzling skillet, then pour the rested batter directly over the center. Return the skillet immediately to the 425°F oven. Bake undisturbed for 18 to 20 minutes until the pancake edges have billowed high above the skillet rim, deeply golden and crisp.
Top and serve immediately: Remove from oven (the dramatic puff will deflate gently after a few minutes, creating a natural bowl). Spoon the remaining warm caramelized apples into the center, dust generously with powdered sugar, and serve warm with maple syrup!
