Preheat the oven to 425°F (220°C). Place your empty oven-safe skillet in the oven while it preheats so it gets very hot.
Make the batter: In a mixing bowl (or blender), combine eggs, milk, flour, granulated sugar, vanilla, cinnamon, and salt. Whisk until very smooth (a few tiny lumps are okay, but smoother is better for puff).
Prep the apples: Slice the apples thinly. Toss with brown sugar, lemon juice, and an extra pinch of cinnamon if you want them extra cozy.
Melt the butter in the hot skillet: Carefully remove the skillet from the oven (use oven mitts!). Add the butter and swirl until melted and foamy.
Pour and bake: Immediately pour the batter into the skillet and return it to the oven. Bake 18–22 minutes, until puffed and deeply golden around the edges.
Cook the apples (while it bakes): In a small skillet (or saucepan), sauté the apple mixture over medium heat 5–7 minutes until softened and glossy. (If you prefer, you can do this step first either way works.)
Serve right away: Spoon warm cinnamon apples over the pancake. Dust with powdered sugar and add maple syrup if desired. Slice into wedges and enjoy while it’s warm and puffy.