Combine the melted butter, brown sugar, and granulated sugar in a medium bowl.
Stir in the egg yolk and vanilla and mix well.
Add the flour, baking soda, and salt, and mix just until combined and no streaks of white remain. The dough will be thick. Fold in the chocolate chips.
Transfer the dough to a sealed container and refrigerate until you're ready to bake. If the dough is very firm, let it sit at room temperature for 15 minutes before cooking.
Prepare a campfire and let it cook down to low flames or very hot coals.
Spray both insides of the pie iron with cooking spray.
Pinch off a 2 tablespoon portion of the dough and shape it into a slightly flattened ball. Place it in the pie iron and close it.
Bake the cookie over the flame or hot coals, flipping occasionally, until the exterior of the cookie is golden brown and crisp, about 10 minutes. The exact timing will depend on the heat of your fire and how close your pie iron is to the heat.
Let the cookie cool for a few minutes before enjoying.