Start by setting your oven to 425°F. Place the butter directly into your cast iron skillet and slide it into the oven so it melts while everything heats up. This step matters because a hot, buttered pan is what makes the batter puff so dramatically once it hits the heat.
While the skillet is heating, whisk your eggs in a medium bowl until they're fully blended. Add the flour, milk, pumpkin purée, sugar, vanilla, cinnamon, nutmeg, and salt, whisking until the mixture is smooth and no lumps remain.
Carefully pull the skillet out of the oven once the butter has melted completely. Pour your batter directly over the melted butter, letting it spread naturally to fill the pan without stirring it again.
Slide the skillet back into the oven and bake for 28 to 30 minutes. You'll see the edges rise dramatically around the 20-minute mark, and the top should turn a deep golden brown by the time it's done.
Pull the skillet out and cut the pancake into wedges right away, since it will start deflating within a minute or two of leaving the oven. Transfer the wedges to plates and add your toppings before it cools.