Start a chimney of coals/briquettes (you'll need 12), or prepare hot embers in a campfire. It will take about 15 minutes for the coals to turn grey and be ready to use. Prepare the cake while the coals are heating.
In a large bowl, whisk the flour, ⅓ cup sugar, ¼ cup cocoa powder, baking powder, cinnamon, and salt together.
Pour in the milk, melted butter, and vanilla, and mix just until no floury streaks remain. Don't over-mix the batter. Scrape it into the prepared Dutch oven.
Add the topping
Sprinkle the chocolate chips, ¼ cup sugar, and 2 tablespoons cocoa powder over the base layer.
Pour the boiling water over the cake. Do not stir.
Cover the Dutch oven and place it in your fire pit with four coals on the bottom and eight coals on the lid. Cook until the cake is puffed and beginning to pull away from the edges, with ribbons of pudding-like chocolate running through the cake, about 30 minutes.
Remove from the fire and cool for 10 minutes. Serve warm.
Notes
Get ahead--Mix the dry ingredients for the base layer and the topping before heading out to camp. Store them in separate resealable bags.
Tuck in the parchment--Ensure that you don't have any bits of parchment sticking out of the Dutch oven as it could ignite.
Protect your pot--Since you're cooking in a Dutch oven outside, the cooking time can vary due to outside temperature and wind. If you're cooking on a very windy day, try to protect the Dutch oven with a barrier, such as a pile of rocks, or a tarp.