Light charcoal briquettes in a campfire or grill. Allow to burn until coals are mostly covered in ash, 15 to 20 minutes.
Spread coals out to flatten and place a camping-safe Dutch oven on top. Add butter and let melt.
Meanwhile, sift flour, baking powder, cinnamon, and salt together in a bowl. Mix in 1 1/2 cups sugar. Add milk and stir until batter is well combined.
Add remaining 1 1/2 cups sugar to peaches in a separate bowl; mix until evenly coated.
Swish Dutch oven around to coat all sides with melted butter. Pour liquid batter into the pot, then pour peach mixture into the center of the batter. Cover with the lid and place hot charcoal on top of the lid.
Cook until done, checking every 20 minutes to prevent overbaking, about 1 hour.