Place oven rack in middle position and preheat to 450℉.
Place a 9- or 10- inch cast iron skillet over low heat on stovetop while you prepare batter.
Mix 2 tbsp of melted butter, milk, flour, eggs, granulated sugar, vanilla, salt, and lemon zest and blend until smooth.
Place remaining 2 tbsp of butter into cast iron skillet. Turn up heat to medium-high. Let butter melt and add blueberries. Shake to coat blueberries in butter.
Immediately pour batter over the blueberries and transfer skillet to oven. Bake until the pancake is puffed and golden, about 15 minutes.
Remove pan from oven, and dust with confectioners sugar (optional). Serve immediately with optional maple syrup.
Notes
Cranberry Orange variation:Substitute 1 ½ cups cranberries for blueberries, and zest of one orange for lemon.