1. Preheat the oven to 450oF and place an oven rack in the middle position.
2. Place a 9- or 10-inch cast iron skillet over low heat on your stovetop. Keep it over low heat while you prepare the batter.
3. Melt 2 tablespoons of the butter then transfer to a blender. Add the milk, flour, eggs, granulated sugar, vanilla, salt, and lemon zest, and blend until smooth.
(Do not skip this step — the batter will be lumpy if you do.)
4. Place the remaining 2 tablespoons of butter into the cast iron skillet. Turn the heat up to medium-high. Let the butter melt, add the blueberries, and give the skillet a shake. Immediately pour the batter over the blueberries and transfer the skillet to the oven. Bake until the pancake is puffed and golden, about 15 minutes.
5. Remove pan from the oven, and dust with powdered sugar, if you wish. Cut and serve immediately.
Cranberry Variation: Use 1.5 cups of cranberries in place of the blueberries and use the zest of one orange in place of the lemon zest.
