Preheat your oven to 325 degrees Fahrenheit.
Butter a 2-quart baking dish using unsalted butter.
Sift together the unsweetened cocoa powder and all-purpose flour in a medium bowl.
In a large mixing bowl, use an electric mixer to beat the granulated sugar and eggs on medium-high speed until pale and light yellow, about 5 to 10 minutes.
Reduce the mixer speed to low and add the vanilla bean seeds along with the sifted cocoa and flour mixture. Mix only until combined.
Still on low speed, slowly pour in the unsalted butter, mixing until just combined.
Scrape the brownie mixture into the prepared baking dish.
Place the baking dish into a larger baking pan and add enough hot tap water to come halfway up the sides of the baking dish.
Bake in the preheated oven for 1 hour.
Remove the baking dish from the larger pan and place it on a wire rack to cool completely.
Serve at room temperature.
Store leftovers, covered, in the refrigerator for up to 3 days.