In a large saucepan, combine the peaches and ¾ cup of the sugar.
Cook over medium heat, stirring often, until the sugar dissolves, the juice is thickened, and the peaches are softened but still firm, 8 to 10 minutes.
In a large bowl, whisk together the flour, remaining 1 cup of sugar, baking powder, and salt.
Stir in the milk and mix until combined.
Place the butter in the cast iron skillet set it in the oven.
Preheat the oven to 350°F. Take out the skillet when the butter has melted, 3 to 5 minutes.
Pour the batter into the skillet on top of the melted butter.
Spoon the peaches and juice over the batter and sprinkle the cinnamon on top.
Bake until a tester inserted in the center comes out clean, about 30-35 minutes.