Place a rack in the center of the oven and preheat the oven to
350 F. Set aside an ungreased 13- by 9-inch baking pan.
Place the cake mix, brown sugar, melted butter, eggs, and vanilla
in a large mixing bowl. Blend with an electric mixer on low speed
for 1 minute. Stop the machine and scrape down the sides of the
bowl with a rubber spatula. Blend again on low speed for 1 minute.
Transfer the batter to the pan, smoothing it out with the spatial
so that it reaches all sides. Sprinkle the cashew pieces and butterscotch
chips over the batter. Place the pan in the oven.
Bake the cake until it is golden brown and a toothpick inserted
in the center comes out clean, 27 to 29 minutes. Remove the pan
form the oven and place it on wire rack to cool for 30 minutes.
Cut the cake into 24 bars. Remove the bars from the pan with a
metal spatula, and serve.
Store the bars, covered in plastic wrap, at room temperature for
up to 1 week. Or freeze them, wrapped in aluminum foil, for up to
6months. Thaw the bars overnight on the counter before serving.