Place empty oven-proof skillet (safe up to 450°) into the oven with rack in middle position (batter will rise well above the pan so ensure there will be room). Preheat oven to 450°.

Combine eggs, milk, flour and salt into a medium bowl. Stir with a fork until smooth and just a few small lumps remain. 

When the oven has reached temperature, remove the skillet and add 2 tbsp butter. The butter will sizzle and melt; swirl the butter in the pan until melted, tilting to coat the sides with melted butter. Add the batter to the pan, and pop back into the oven. Bake 15 minutes, or until very puffy and golden. 

While the pancake is in the oven, melt 2 tbsp butter in a medium skillet on the stove top, over medium high heat. Add apple slices and stir, turning to coat evenly with butter. Add brown sugar and cinnamon. Cook on high for 10–12 minutes, until the apples soften and the juices have combined with the brown sugar and cinnamon and cooked down into a syrupy sauce.

Remove skillet from the oven, slice pancake and sprinkle with powdered sugar on top. Add sautéed apples to the plate, spooning the sugary juices on top of the pancake.