Heat the oven to 325 degrees F. Line an 8x8-inch baking pan (preferably metal) with foil across the bottom and up two opposite sides of the pan.

In a medium bowl, thoroughly mix the cocoa, sugar, vanilla, salt, and hot melted butter with a hand mixer or with vigorous strokes of a whisk. Add the eggs, one at a time, beating until the batter is thick and lightened in color, 1 to 2 minutes. Add the flour and stir with a rubber spatula just until blended. Fold in the nuts, if using. Spread the batter evenly in the prepared pan and bake until the top is puffed and slightly crusted and a toothpick inserted in the center comes out with a little fudge batter clinging to it, 28 to 30 minutes.

Let the brownies cool completely in the pan on a rack. Lift the ends of the foil to remove them. Invert the brownies on a tray and peel off the foil. Turn the brownies right side up on a cutting board and cut into squares (wipe the knife between each cut, as the brownies will be soft and sticky).