Build a campfire and allow it to develop hot embers.
Cut each peach in half and remove the pit. Leave the skin on.
Lay both peach halves cut-side up on a large sheet of aluminum foil.
Sprinkle cinnamon over both halves. Add brown sugar if desired.
Place one large marshmallow into the hollow of one peach half.
Press the two peach halves back together so the marshmallow is sandwiched inside.
Wrap the peach tightly in the foil, sealing all edges. Repeat for each peach.
Place wrapped peaches directly into the campfire. Cook for 3–4 minutes.
Remove carefully using tongs - the foil will be very hot.
Allow to cool for 30 seconds, then unwrap and enjoy immediately.