Preheat oven to 325 degrees F (165 degrees C). Lightly butter a 9x13-inch baking dish. Sprinkle cocoa powder into the dish.
Melt chocolate and butter in top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching, about 5 minutes.
Whisk eggs, brown sugar, and demerara sugar together in a bowl until smooth. Stir chocolate mixture into egg mixture until thoroughly mixed. Stir in flour, vanilla extract, and salt into chocolate-egg mixture until thoroughly mixed; pour into the prepared baking dish.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 35 minutes.
