1. Preheat oven to 400 degrees. Place 2 tablespoons of butter into a 10-12 inch cast iron skillet and place it into the oven for 8 minutes, allowing the butter to melt and the pan to preheat.
2. Meanwhile, melt the other 1 tablespoon of butter. To a blender, add the 1 tablespoon melted butter, eggs, egg white, milk, sugar, lemon zest, vanilla, flour, and salt. Whirl until mixture becomes a smooth batter, about 30 seconds.
3. When the cast iron skillet is preheated and the butter is melted, carefully remove the hot pan from the oven and pour in the pancake batter. Return the pan to the oven and bake for 20 minutes, until puffed up and golden. Remove the pan from the oven, and working quickly (as the pancake rapidly deflates) top with powdered sugar a generous squeeze of lemon, slice and serve with more powdered sugar and lemon wedges on the side.
When Meyer lemons aren\'t in season, a traditional lemon can be used.