Preheat oven to 425°F. Grease skillet. Add butter to skillet and pop into the oven until butter melts, about 3 minutes. Remove from oven.
Add blueberries to the skillet.
Meanwhile, whisk together eggs, milk, sugar, vanilla, flour, and cinnamon until smooth.
Pour batter into skillet over blueberries.
Bake for 20 minutes, until center is set. If using fresh berries, bake for 18 minutes.
Sprinkle with powdered sugar if using and serve immediately.
Notes
Leftovers can be stored in an airtight container in the fridge for up to 3 days. Reheat in the microwave until heated through, refresh with powdered sugar.