1. Preheat oven to 350°F. Line a 9-by-13-inch baking pan with foil, leaving an overhang on narrow ends.
2. Melt butter with both chocolates in a large heatproof bowl set over a pot of almost simmering water. Remove and let cool slightly.
3. In a separate bowl, whisk eggs with both sugars until smooth. Whisk in chocolate mixture. Stir in salt, cinnamon and cayenne. Scrape down sides of bowl and whisk until well blended. Mix in flour. Transfer batter to prepared pan and smooth top. Bake until center is set, edges are puffy and brownies are just beginning to pull away from sides of pan, about 25 minutes. Transfer to a rack and let cool.
4. Cover brownies and refrigerate for at least 30 minutes or up to overnight. Using foil, lift slab of brownies out of pan. Peel off foil, place slab on a large cutting board and cut into 48 pieces, wiping off knife in between cuts.