Preheat oven to 325 degrees F. Butter a 2-quart baking dish; set aside. Sift together the cocoa powder and flour into a medium bowl; set aside.
Using an electric mixer, beat the sugar and eggs on medium-high speed until very pale and light yellow, 5 to 10 minutes. Reduce the mixer speed to low and add the vanilla bean seeds and the cocoa mixture, mixing only until combined. Still on low speed, slowly pour in the melted butter and mix again until just combined.
Scrape the brownie mixture into the prepared baking dish, and place into a larger baking pan. Add enough hot tap water to come halfway up the sides of the baking dish. Bake for 1 hour, at which point a tester inserted 2 inches from the side of the pan will come out with some moist crumbs still attached and the center will appear under-baked. Remove the baking dish from the larger pan and place on a wire rack to cool completely. Serve at room temperature. Leftovers should be stored, covered, in the refrigerator for up to 3 days.