Place cake mix, brown sugar, melted butter, eggs and vanilla in large bowl of electric mixer. Beat on low speed 1 minute. Stop machine; scrape down sides of bowl with rubber spatula. Beat 1 minute more. Stir in half the cashews and butterscotch chips.
Transfer batter to ungreased 11 3/4 x 7 1/2 x 1 3/4 inch glass baking dish. Spread batter evenly to all sides using rubber scraper. Sprinkle remaining cashew pieces and butterscotch chips over batter.
Bake until golden brown and wooden pick in center comes out clean, about 30 to 32 minutes. Transfer pan to wire rack to cool completely.
Cut cake into 24 bars. Remove bars from pan with metal spatula.