In a bowl, sift together the flour, baking powder, baking soda and salt. Set aside.
In a large mixing bowl, combine melted butter and brown sugar. Using a hand mixer, beat at low speed for 1 minute.
Switch to medium speed and beat for 1 more minute. Add the eggs and beat at low speed for 30 seconds. Add the vanilla flavor and beat at medium speed for 30 seconds.
Add the flour mixture and half of the chopped cashew nuts; mix at low speed for 1 minute. Scrape the bottom and the sides of the bowl.
Warm the corn syrup in the microwave for 1 minute and then add to the mixture. Mix at low speed until well incorporated.
Pour the batter onto a greased 9”X 13” baking pan. Sprinkle the remaining cashews and chocolate chips, if using.
Bake for 40-45 minutes or until a cake tester comes out clean.
Let the butterscotch cool for 10 minutes before removing from pan.