Get Your Oven Hot: Preheat your oven to 425°F (220°C). Make sure it's good and hot!
Prep the Batter: In a medium bowl or blender, whisk (or blend) together the room temperature eggs, milk, flour, vanilla extract, lemon zest, sugar, and salt until smooth. Don't overmix, just combine until there are no lumps. Let the batter rest for a few minutes while you heat the skillet – this allows the flour to hydrate a bit, which can help with the puff.
Heat the Skillet: Place the 3 tablespoons of butter in a 10-inch cast iron skillet (or a heavy oven-safe skillet of similar size). Put the skillet into the preheating oven. Let the butter melt completely and get nice and bubbly, about 3-5 minutes. The skillet needs to be screaming hot!
Add Batter and Blueberries: Carefully remove the hot skillet from the oven (use oven mitts!). Pour the batter directly into the center of the hot, melted butter. Immediately scatter the blueberries evenly over the batter.
Bake! Place the skillet back into the hot oven. Bake for 15-20 minutes, or until the edges are dramatically puffed up and golden brown, and the center is set. Avoid opening the oven door while it's baking if you can – peeking can make it fall!
Serve Immediately: Carefully remove the skillet from the oven. Your Dutch baby will start to deflate almost immediately (that's totally normal!). Serve it right away while it's still warm and puffy around the edges.
Top It Off: Dust generously with powdered sugar, add spoonfuls of lemon curd, or drizzle with maple syrup. Cut into wedges and enjoy!