In a bowl whisk together the whole milk, eggs, maple syrup, and vanilla extract. Add the flour and salt and whisk in until there is no dry flour. Set aside.
Heat the oven to 425F. Place a 12 inch cast iron skillet in the oven as it heats.
Add the butter and blueberries to the skillet and put it back in the oven for 1-2 minutes, or until the butter is fully melted. Swirl the skillet to make sure the whole bottom is coated with butter.
Pour the batter into the skillet and give it a stir to distribute the berries.
Bake the Dutch baby for 20 minutes.
Remove from the oven and dust with confectioners' sugar. Other toppings can include blueberry (or other) jam, whipped cream, lemon juice, vanilla Greek yogurt, and ice cream.
Notes
Make ahead: You can make the batter the night before and store it in the refrigerator. Allow it to come to room temperature before pouring it in the skillet.
Skillet size: If you have a 10 inch skillet, reduce the amounts by about 1/3. So it would be 2 eggs, 1/2 cup flour, and 1/2 cup milk for the base.