Position rack in center of oven; preheat to 350°F. Line an 8-inch-square baking pan with foil, letting it overhang on two opposing sides. Coat with cooking spray.

Sift flour, confectioners’ sugar and cocoa together into a small bowl. Combine the 3 ounces coarsely chopped chocolate and oil in a heavy medium saucepan; place over the lowest heat, stirring, until just melted and smooth, being very careful the chocolate does not overheat. Remove from the heat and stir in granulated sugar, corn syrup mixture, vanilla and salt until the sugar dissolves. Vigorously stir in egg until smoothly incorporated. Gently stir in the dry ingredients. Fold in the walnuts (if using) and the remaining 2 1/2 ounces chopped chocolate just until well blended. Turn out the batter into the pan, spreading evenly.

Bake the brownies until almost firm in the center and a toothpick inserted comes out with some moist batter clinging to it, 20 to 24 minutes. Let cool completely on a wire rack, about 2 1/2 hours.