Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.
In a saucepan, melt butter over low heat. Remove from heat and stir in both sugars.
Beat in eggs and vanilla, then stir in cocoa powder, flour, and salt until just combined.
Pour batter into prepared pan and smooth the top.
Bake for 25–30 minutes, or until a toothpick inserted comes out with moist crumbs.
While brownies bake, make the caramel: In a small saucepan, combine brown sugar, butter, milk, and salt. Bring to a simmer over medium heat and cook for 3–4 minutes. Remove from heat and stir in vanilla.
When brownies are out of the oven, immediately top with popcorn.
Drizzle warm caramel over the popcorn to coat evenly. Use a spatula to gently press popcorn into caramel and brownie layer.
Let cool at room temperature for at least 30 minutes before slicing.