Place a 10-inch cast iron skillet on the center rack of your oven and preheat to 425°F (220°C). Heating the skillet thoroughly is essential for maximum batter rise!

In a blender or food processor, combine eggs, room-temperature milk, all-purpose flour, granulated sugar, vanilla extract, and salt. Blend on high speed for 30 seconds until completely smooth and frothy. Let the batter rest for 10 minutes at room temperature.

In a separate medium skillet over medium heat, melt 3 tablespoons of butter. Add sliced apples, brown sugar, cinnamon, nutmeg, and a pinch of salt. Sauté for 5 to 6 minutes until the apples are tender and coated in a glossy caramel glaze. Remove from heat.

Carefully remove the blazing hot cast iron skillet from the oven using thick oven mitts. Drop in the remaining 2 tablespoons of butter and swirl until completely melted, foaming, and coating the bottom and sides.

Immediately pour the blender batter into the center of the hot skillet. Scatter half of the caramelized apples across the batter.

Return the skillet to the oven and bake undisturbed for 18 to 20 minutes until the edges are dramatically puffed, crispy, and deeply golden brown. (Do not open the oven door during baking!).

Remove from the oven, top with the remaining warm caramelized cinnamon apples, dust generously with powdered sugar, and serve immediately with warm maple syrup!