Preheat the oven to 325 degrees, making sure that there is a rack in the bottom third. Lightly grease the pan, adding parchment paper or foil to make removal easier, if you wish. Combine the butter, sugar, cocoa, and salt (if using unsalted butter) in a medium heatproof bowl. In either the microwave or a double broiler, stir occasionally until the butter is completely melted, the mixture is just a little gritty and the mixture is warm, but not hot. If the mixture gets too hot, allow to cool to warm before continuing. Stir in the vanilla. Add the eggs one at a time, mixing in each until fully incorporated. Add the flour and stir until flour is fully incorporated. Continue stirring until the batter becomes thick and smooth. At this point, the mixture should be easy to cleanly scrape from the bowl with a spatula. Stir in the nuts, if using. Spread evenly in the lined pan. Bake until a toothpick or cake tester stuck into the center comes out slightly moist. All of the butter should be fully absorbed, not resting on the top, about 20-30 minutes depending on your oven. Watch it closely after 20 minutes because the butter is absorbed quickly! Let cool completely in the pan. Chill in refrigerator for clean cutting lines, if desired. Lift up the ends of the parchment or foil liner, if using, and remove brownies from pan to cut and serve.