Preparation Preheat oven to 325°F. Grease 9-inch square baking pan. Stir butter and chocolate in heavy medium saucepan over medium heat until melted. Whisk eggs and sugar in bowl. Add warm chocolate mixture and vanilla; whisk to blend. Whisk in flour. Add nuts. Pour into prepared pan. Bake until tester inserted into center comes out with moist crumbs attached, about 25 minutes. Cool in pan on rack. Cut into squares. (Can be made 1 week ahead. Store chilled in airtight container. Serve at room temperature.)