Heat oven to 425℉.
Spray a 10 inch cast iron skillet with cooking spray.
Wipe clean.
Place skillet in oven.
In a medium bowl, whisk together flour, sugar, and salt.
Add eggs, milk, and vanilla.
Whisk until smooth.
Set aside for 20 minutes.
After resting time is done, add butter to pan in oven.
Allow to melt.
Carefully remove pan from oven.
Pour batter into hot pan.
Return to oven.
Bake for 10 minutes.
Reduce oven temperatureto 350℉.
Bake for another 8-10 minutes, until puffy.
Remove from oven.