Preheat oven to 325°F. Add butter to cast iron skillet and melt.
In a bowl, whisk flour, sugar, and milk until smooth.
Pour batter over melted butter (do not stir).
Add canned peaches with syrup and fresh peaches on top.
Sprinkle cinnamon.
Bake 1 hour.
If adding extra peaches, add ½ batch extra batter and bake 10 more minutes at 375°F.
Cool 10–15 minutes before serving.