Set nuts aside. In order listed, measure remaining ingredients except chocolate pieces into blender. Blend 30 seconds on high speed, stopping blender occasionally to scrape sides with rubber spatula. (Batter may be slightly lumpy). Pour batter into pan. Bake about 35 minutes or until wooden pick inserted in center comes out clean. Immediately sprinkle chocolate pieces over cake. Place baking sheet over pan so contained heat will soften chocolate pieces. Spread softened chocolate evenly over cake; sprinkle with reserved nuts. Serve warm. To prepare cake with electric mixer, measure all ingredients except nuts and chocolate pieces into large mixer bowl. Blend half minute on low speed, scraping bowl constantly. Beat 3 minutes high speed, scraping bowl occasionally. Bake as directed. *If using self-rising flour, omit baking powder and salt.