2 sticks (16 Tbsps) butter, more for pan and parchment paper
8 ounces bittersweet chocolate
4 eggs
1/2 teaspoon salt
1 cup dark brown sugar, such as muscovado
1 cup granulated sugar
2 teaspoons vanilla extract
1 cup flour
1/2 cup chopped walnuts or
3/4 cup whole walnuts, optional