10 tablespoons (1 1/4 sticks) unsalted butter
1 1/4 cup sugar
3/4 cups plus 2 tablespoons unsweetened cocoa powder (natural or Dutch-process)
1/4 teaspoon salt
1/2 teaspoon pure vanilla extract
2 cold large eggs
1/2 cup all-purpose flour (unsifted, measured by stirring briefly, spooning into the measuring cup until it's heaped above the rim, then leveling it with a straight-edged knife or spatula -- it should weigh nearly 2.5 ounces)
2/3 cups walnut or pecan pieces (optional)