Preheat oven to 350°F. Butter an 8-inch square baking pan and line the bottom with parchment paper, allowing the paper to hang over the edge of the pan by 2-inches (this will help you get the brownies out of the pan). Butter the bottom of the paper.
Fill a medium saucepan with a few inches of water. Set over medium heat and bring to a simmer. Reduce heat to low so the water is barely simmering.
Place the ½ cup butter and chopped chocolate in a double boiler or small metal bowl that fits over the saucepan. Set over the barely simmering water and heat,stirring occasionally, just until melted. Remove from heat and set aside to cool slightly.
Place the sugar, salt, vanilla and eggs in the bowl of an electric mixer fitted with the paddle attachment. Beat at medium-high speed until combined, about 30 seconds. (Alternatively, you can whisk them together by hand until well combined.)
Turn the mixer off and add the melted chocolate mixture (or add it while whisking by hand); and continue mixing until well combined. Add the flour and mix just until incorporated.
Stir in the nuts (if using) and pour the batter into the prepared baking pan,spreading evenly with a spatula. Bake for about 25 to 30 minutes, or until the edges are puffed, the top is crackled and a toothpick inserted in the center comes out clean (with a few wet crumbs clinging to it but no streaks of batter). If it’s not done, continue to bake, checking the brownies every five minutes for doneness.
Remove pan from the oven and set on a wire rack. Let brownies cool completely in the pan. Grasp the ends of the parchment paper and lift the brownies out. Cut into thirds lengthwise and crosswise to make 9 roughly 3-inch squares. Serve alone or with ice cream.