Make Pancake Batter: In a medium bowl, whisk together the eggs and milk until smooth. Add the flour, baking powder, vanilla, and a pinch of salt. Whisk until combined, or blend everything in a blender until smooth. Let the batter rest for 30 minutes while you prepare the oven and apples, this helps it puff up beautifully in the oven.
Preheat Oven: Preheat your oven to 425°F. Place your oven proof skillet inside while it heats so it’s nice and hot for baking. When the oven is ready, carefully remove the skillet. A hot pan helps the pancake rise and prevents sticking.
Prepare Apples: Peel, core, and slice the apples into ¼-inch slices. In a small bowl, toss them with cinnamon, nutmeg, and sugar until evenly coated.
Prepare Cast Iron Skillet: Using oven mitts, carefully remove the hot skillet from the oven. Add the butter and swirl it around to coat the bottom and sides completely. This not only prevents sticking but adds a golden buttery edge to the pancake.
Make Pancake: Pour in enough batter to just cover the bottom of the skillet, about half of the mixture. Arrange the apple slices in a single, slightly overlapping layer. Pour the remaining batter over the top, ensuring all the apples are covered.(Be careful the skillet will be very hot!)
Bake: Bake the pancake for 10 minutes at 425°F, then reduce the heat to 400°F and continue baking for 12–15 minutes more, until puffed, golden, and set in the center. If any apples peek through, they’ll caramelize slightly — that’s part of the charm. Slice and serve immediately while hot, with a drizzle of maple syrup or a dusting of powdered sugar.