Place your 10-inch cast iron skillet in the oven and preheat to 425 degrees. Let the skillet heat for at least 15 minutes. A fully hot pan is what creates the dramatic puff, so do not rush this step.

While the skillet heats, add the eggs, milk, flour, granulated sugar, vanilla, and salt to a blender. Blend on high for 30 to 45 seconds until the batter is completely smooth and a little frothy. Set aside.

Using oven mitts, carefully pull the hot skillet out of the oven and set it on the stovetop over medium heat. Add 2 tablespoons of the butter. It will melt fast. Add the apple slices, brown sugar, and cinnamon. Stir and cook for 3 to 4 minutes until the apples are just tender and coated in a light caramel glaze. Spread them into an even layer.

Add the remaining 1 tablespoon of butter to the skillet and let it melt over the apples. Working quickly, give the batter one more quick swirl and pour it evenly over the apples. Immediately slide the skillet back into the hot oven.

Bake for 18 to 22 minutes without opening the oven door. The pancake is done when the edges are deep golden brown and puffed up dramatically, with a slightly custardy center that is set but not wet. It will deflate within a minute or two of coming out of the oven, and that is completely normal.

Dust generously with powdered sugar and serve immediately, scooping portions straight from the skillet at the table.