Preheat oven to 375 degrees F.
Place 2 tablespoons of the melted butter into a 10-inch cast iron skillet and place in the oven. Set the remaining tablespoon of melted butter aside to cool slightly. Wait 10 minutes before assembling the other ingredients.
Place the flour, vanilla sugar, salt, milk, eggs and remaining tablespoon of melted butter into the bowl of a food processor or blender and process for 30 seconds. Carefully pour the batter into the preheated skillet. Bake on the middle rack of the oven for 30 to 35 minutes or until the edges are puffed and brown. Sprinkle with additional vanilla sugar and serve with lemon wedges.