Preheat the oven to 350 °F (175 °C). Butter a 9x9 brownie pan, fit the base with parchment, and sprinkle the sides with cocoa powder.
Melt the chocolate with the butter in a small saucepan, then set aside to cool.
Sift together the flour, cocoa, and salt.
In the bowl of an electric mixer fitted with the paddle attachment, beat together the eggs, sugar, and vanilla. Add the sifted dry ingredients, then the melted chocolate mixture.
Dump in the ground and chopped brittle (or fold it in to avoid crushing the chunks of popcorn brittle). Transfer the brownie batter to the prepared pan.
Bake the brownies in the centre of the oven until a cake tester inserted in the middle comes out quite clean (about 30 to 40 minutes).