Preheat your oven to 400°F (200°C).
In a blender, combine the milk and eggs. Blend for about 30 seconds until well mixed and slightly frothy.
Add in the flour, 2 tablespoons of sugar, lemon zest, salt, and vanilla. Blend again until the batter is smooth, about 30 seconds more. Allow the batter to rest for about 10 minutes.
Combine the remaining 1 tablespoon of sugar and the remaining lemon zest in a small bowl. Rub the zest into the sugar until fragrant. Toss the blueberries in this mixture, coating them gently.
Slice the butter into pieces and place them in a 10-inch cast iron skillet. Place the skillet in the preheated oven for about 2 minutes or until the butter is melted and bubbling.
Carefully remove the skillet from the oven. Swirl or brush the melted butter around the edges of the pan.
Pour the batter into the skillet over the melted butter. Sprinkle the coated blueberries evenly over the batter.
Bake in the oven for 18 to 20 minutes, or until the Dutch baby is puffed and golden brown around the edges.
Once baked, remove the skillet from the oven and let it cool for a minute. Slice into wedges and serve warm.
