Place butter into a 12-inch cast iron skillet. Put the skillet in your oven while it's preheating. The butter will melt and the skillet will get hot.
In a medium mixing bowl, whisk together the eggs, milk, flour, sugar, vanilla, and salt until smooth. The batter will be pretty liquidy.
Once the oven is preheated, carefully pull out the oven rack your skillet is on. Pour the batter into the hot skillet.
Push the rack back in, close the oven, and bake for 20 to 25 minutes. The edges will puff up above the skillet and the center will be puffed and golden.
Remove from the oven and let sit for 2-3 minutes so the pancake can settle.
Cut the pancake into slices. Sprinkle generously with powdered sugar and serve with lemon slices for squeezing, or top with butter and syrup.
Notes
Storage: Allow leftovers to cool completely, then store in an airtight container in the fridge for up to 2-3 days.Freezing: To freeze, wrap individual pancakes or pieces in plastic wrap, then place in a plastic freezer bag.