Preheat oven to 400 degrees, and place cast iron pan inside.
Whisk together milk, eggs, and vanilla.
Sift in flour, salt, cinnamon, and sugar until all the lumps are gone.
Carefully remove hot pan from oven, place butter inside, and swirl around for 1-2 minutes, until butter is melted and beginning to smell slightly nutty. Pour excess melted butter into another dish.
Drizzle butter into batter slowly while whisking. Pour batter into cast iron skillet. Sprinkle with blueberries, and place into heated oven for 15-20 minutes, or until puffed and golden brown.