Prepare your campfire or camp stove: If using a campfire, let it burn down to a bed of even, glowing coals (no flames). If using a camp stove, set it to low-medium heat. You'll need a stable grate or trivet that holds the skillet 4-6 inches above the heat source.
In a 10-inch cast iron skillet over low heat (or a few coals), melt 1/4 cup butter. Once melted, sprinkle the brown sugar evenly over the butter. Stir gently with a wooden spoon until the sugar dissolves into a syrupy paste, about 1-2 minutes. Remove from heat.
Arrange the fresh pineapple rings in a single layer over the brown sugar mixture, overlapping slightly if needed. Place a cherry half in the center of each ring, cut side up. Set the skillet aside.
In a medium bowl, whisk together the flour, granulated sugar, baking powder, and salt. In a separate small bowl, whisk the milk, 1/4 cup melted butter, vanilla extract, and egg until smooth. Pour the wet ingredients into the dry and stir just until combined — a few lumps are fine. Do not overmix.
Gently spoon the batter over the pineapple rings, spreading it evenly to the edges. The batter will be thick; that's normal.
Cover the skillet tightly with a lid or a double layer of heavy-duty aluminum foil, crimped around the edges. Place the skillet on the grate over the coals or stove. If using a campfire, place a few coals on top of the lid (or foil) to create even top heat. This is the trick for even baking.
Bake for 30-35 minutes, rotating the skillet 90 degrees every 10 minutes to prevent hot spots. The cake is done when a skewer inserted into the center comes out clean, and the top springs back to the touch. If the top is browning too fast, move the skillet to slightly cooler coals or reduce stove heat.
Remove the skillet from heat. Let it rest for 5 minutes — this allows the caramel to set slightly. Place a large plate or cutting board over the skillet, then carefully invert the cake in one swift motion. Lift the skillet off. Replace any pineapple rings that stuck to the skillet. Serve warm or at room temperature.