Heat oven to 425°. Put 1 tablespoon of butter in a large cast-iron skillet or pie plate and place in oven. Whisk eggs in a large bowl with milk and vanilla. Add flour, salt, and lemon zest and whisk until combined. Whisk in melted butter. Carefully remove hot pan from oven and pour the batter into the skillet. Sprinkle with blueberries. Bake for 15 minutes.
Reduce heat to 325° and bake an additional 10 minutes. Remove from oven and dust with powdered sugar and a squeeze of fresh lemon juice (optional). Serve as is, or with real maple syrup.
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