Preheat oven to 325. Line 11x9 pan with foil and butter it. Melt butter and chocolate in large saucepan over low heat. When melted, remove from heat. Stir sugar into chocolate, then eggs and vanilla(mixture will probably look curdled).
Add flour, salt, nuts; stir until just mixed. Pour batter into pan. Bake brownies until top is golden but center is still soft, 30-35 minutes. Cool and then refrigerate at least 3 hours, then cut.