Preheat oven to 180°C. Grease 33x23cm cake pan with butter. Line base and long sides with baking parchment paper.
Place cake mix in large bowl; remove 1 tablespoon of the dry mix to smaller bowl, and stir in chocolate chips to coat. Set aside.
Add eggs, oil and water to cake mix in large bowl. Using electric mixer, beat on low speed until moistened, and then beat on medium speed 2 minutes. Fold in chocolate chips mixture and pistachio nuts.
Pour batter into cake pan; spread evenly.
Bake 44 to 46 minutes or until golden and toothpick inserted in center of cake comes out clean. Cool in pan 10 minutes, and then use parchment paper to carefully lift cake out onto cooling rack.
Beat whipping cream mix and cold milk in medium bowl as directed on package.
Cut warm cake into squares. Drizzle with chocolate sauce, and sprinkle with chopped pistachio nuts. Serve with whipped cream.