In a medium bowl, gently toss the blackberries with 1/2 cup sugar, cornstarch, lemon juice, and cinnamon until evenly coated. Set aside.
In a separate bowl, whisk together the flour, 2 tablespoons sugar, baking powder, and salt. Cut in the cold butter using a pastry cutter or your fingertips until the mixture resembles coarse crumbs.
Stir in the milk and vanilla extract just until a soft dough forms. Do not overmix.
Place a small wire rack or a few clean pebbles in the bottom of a 10-inch Dutch oven. Pour the blackberry mixture into the Dutch oven.
Spoon the biscuit dough over the berry filling in 6 even dollops. Do not spread it out — it will expand as it bakes.
Cover the Dutch oven with its lid and place it on a camp stove over medium-low heat. Cook for 20 minutes, then rotate the lid a quarter turn to redistribute heat. Cook for another 15 minutes, until the topping is golden and the filling is bubbly.
Remove the Dutch oven from the heat and let it rest for 5 minutes. Serve warm, directly from the pot.