Preheat the oven and the skillets. Place cast iron skillets in an oven and heat to 425°.  

Make the batter. While the oven is heating place flour, eggs, milk, sugar, vanilla and salt in a blender.  Blend for 15 seconds then scrape down the sides to ensure all the flour is incorporated. Blend for an additional 15- 20 seconds until batter is smooth. Let batter rest in the refrigerator while the oven finishes heating.  

Butter the skillets. Remove skillets from oven, add butter to each skillet and swirl the butter in the skillet to coat.  

Cook the pancakes. Pour the batter into a measuring cup and divide evenly between the two skillets.  Bake for 15 minutes or until golden brown and puffed.  

Remove from oven and top with cheese, ham and avocado.  Serve immediately.