Step 1Line a 13-inch by 9-inch baking pan with foil. Lightly grease the foil.
Step 2In a heavy-bottom saucepan, combine the butter and sugar over low heat, stirring frequently until the butter is fully melted to form a smooth syrup. Stir in the cocoa and remove from heat.
Step 3Remove the mixture to a large bowl. Stir in the cream cheese, pressing any bits against the bowl to smooth them out and incorporate into the mixture. Add the eggs, one at a time, until fully combined, then stir in the salt. Add the flour and stir vigorously until the flour is fully incorporated and pulls away from the sides of the bowl.
Step 4Stir in 6 ounces chopped chocolate, along with the nuts, if using. Spread the mixture into the prepared pan. Cover and refrigerate until well-chilled, preferably overnight, before baking.
Step 5Heat the oven to 325 degrees and place a rack on the lower third of the oven. Uncover the brownies and bake just until a toothpick inserted in the center comes out slightly moist, 35 to 40 minutes. Remove from heat and cool completely on a rack.
Step 6Remove the foil-lined brownies from the pan and spread the foil out on all sides to form a border around the brownies (this will keep your countertop clean as you decorate). Lightly cover the cooled brownies with powdered sugar.
Step 7Over a double boiler, or using a microwave-safe container, melt the remaining 2 ounces chocolate. Stir in the cream to form a ganache. Place the ganache in a piping bag, or a sealable plastic bag with one of the corners snipped off, and drizzle the ganache over the brownies. Cut and serve immediately. The brownies will keep, covered and refrigerated, for up to 1 week.