Preheat the oven to 325 F. Lightly coat a 9x13 inch metal baking pan with cooking spray and then line it with parchment paper, with 2 inches of the paper overhanging the edges of the pan.

Combine the flour, cocoa powder, baking powder, and salt in a medium bowl. Using a balloon whisk, vigorously stir the dry ingredients together until they are uniform in color and well blended. Place the unsweetened chocolate and butter in a large microwave-safe bowl and microwave for 45 seconds. Stir and microwave in 30-second intervals until the chocolate mixture is completely melted.

Add the sugars to the chocolate mixture and beat with a balloon whisk by hand for about 30 seconds. Add the eggs one at a time, beating to incorporate between each one. Add the vanilla and beat to incorporate. Add the dry ingredients and, using a large spatula, fold them in until just incorporated and there are no more dry pockets. Fold the bittersweet chocolate into the batter.

Scrape the batter into the prepared pan and spread evenly. Bake until a toothpick inserted in the center comes out with some crumbs attached, 30 to 35 minutes. Let cool completely on a wire rack before adding the caramel popcorn.

Place the oil and salt in a large Dutch oven or pot. Add 3 popcorn kernels to the pot, cover with the lid slightly ajar to let steam release, and turn the heat to high. Cook, shaking the pot occasionally until the kernels pop. Remove the pot from the heat and add the rest of the popcorn kernels. Shake the pot to coat with the oil and salt and wait 30 seconds. Return the pot to the stovetop and reduce the heat to medium-high. Cook, with the lid slightly ajar and shaking the pot occasionally, until there is a 2-second interval between popping. Remove from the heat and pour the popcorn into a large heatproof bowl.

Combine the granulated sugar and corn syrup in a large pot, preferably one with a silver or light-colored bottom (so you can see the color of the sugar). Turn the heat to medium-high. As the sugar starts to melt and turn brown on the bottom and at the edges, stir with a wooden spoon or paddle to encourage the unmelted sugar to melt and caramelize. Continue to cook until all the sugar has reached the color of an old penny. Remove from the heat and add the cream and butter. The caramel will steam and bubble up, so do this carefully. Place back over medium-high heat and cook, stirring to dissolve any hardened bits, until the caramel reaches 245F (firm ball stage). Stir in the kosher salt.

Using an offset spatula or butter knife, quickly spread a thin layer of caramel over the brownie, making sure the caramel reaches the edges. Pour the rest of the caramel over the popcorn, tossing the popcorn to coat with caramel. Pour the still-warm caramel popcorn onto the brownie, and pat the popcorn down with a heatproof spatula or with a piece of parchment or waxed paper to create a layer of popcorn over the brownie. Sprinkle with the finishing salt and let cool to harden.

Once the popcorn has cooled and hardened, remove the entire brownie from the pan by grabbing the overhanging parchment paper and lifting straight up. Transfer to a large cutting board and, using a sharp serrated knife, cut and serve.