Move the oven rack to the middle position and preheat the oven to 450 degrees F. Brush the bottom and sides of a large 12-inch cast iron skillet (or heavy-bottomed skillet) with 2 tablespoons of vegetable oil. Put the skillet in the oven and heat until the oil is shimmering, about 8-10 minutes.
MAKE THE BATTER:
In a large bowl whisk together the flour, cornstarch, lemon zest, and salt.
In a medium bowl whisk the eggs until they are frothy, for about 1-2 minutes. Whisk in the milk, sugar, butter, and vanilla into the eggs until the eggs turn light yellow and no longer stringy, about 1 minute.
Slowly whisk the egg mixture into the flour mixture until combined and smooth. Then, whisk vigorously for 1 minute.
BAKE THE PANCAKE:
Quickly and carefully remove the pan from the oven and swirl the oil around to coat the pan one more tie. Then, pour the batter into the preheated skillet. Bake until the pancake edges are deep golden brown and crisp, about 18-22 minutes.
GARNISH AND SERVE THE PANCAKE:
Use a large spatula to move the entire pancake to a wire rack. Let the pancake cool for 3 minutes. Then, sprinkle the pancake with lemon juice and dust with powdered sugar.
Slice the pancake into wedges and serve with fresh berries and whipped cream (both optional).