Preheat the oven to 400 degrees F.
Spread a tablespoon of butter on two 6-inch cast iron skillets (you can also use 8-inch cast iron skillets). Place the skillets in the preheating oven.
In a blender add the flour, sugar, cornstarch, and salt. Pulse to combine. Add the eggs and blend until it forms a paste. Scrape the sides of the blender as needed.
In a small bowl, mix together the milk, remaining butter, and vanilla. Pour the milk mixture into the blender and blender until everything is fully incorporated - about 2 minutes on high, scraping the sides as needed.
Take the skillets out of the oven (taking care to remember to use an oven mitt). Evenly divide the batter between the skillets and promptly place back in the oven. Bake for about 18 - 20 minutes, or until the edges are crisp and the pancake is puffy.
While the dutch baby pancakes are baking, whip the mascarpone, Bonne Mann strawberry preserves, powdered sugar, and vanilla extract together until light.
Top the finished Dutch pancakes with the whipped mascarpone and fresh strawberries, and more preserves as desired.