In a saucepan over medium heat, melt the bittersweet chocolate, unsweetened chocolate, and butter. Stir frequently until smooth and melted. Remove from heat. Stir in the sugar and vanilla extract. Add the eggs one at a time, whisking after each. Add the flour and salt and stir until just combined. Pour into the prepared pan.

Bake until set but still soft in the center, 25-30 minutes. Cool completely.

To make the caramel popcorn, combine the butter, brown sugar, and corn syrup in a sauce pan over medium heat. Stir to dissolve sugar. Once butter is completely melted, increase the heat and bring the mixture to a boil. Reduce heat and simmer for 10 minutes, until the mixture turns a dark caramel color. Remove from heat and add the sweetened condensed milk, salt, and vanilla extract. Stir until combined.

Spread the popcorn on a sheet pan and slowly pour the caramel over the top. Use a wooden spoon to stir the popcorn and make sure the caramel is distributed evenly. Spoon the caramel popcorn on top of the cooled brownies. Let sit for about 15 minutes, then use a sharp knife to cut into squares.

The brownies are best the day they are made, but can be stored at room temperature in an airtight container for a few days.